🕷️ Crawler Inspector

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Raw Queries and Responses

1. Shard Calculation

Query:

Response:

Calculated Shard: 53 (from laksa165)

2. Crawled Status Check

Query:

curl -X POST \
  'http://laksa053.int.ahrefs:8124/' \
  -H 'Content-Type: text/plain' \
  -H 'X-ClickHouse-Database: crawler3' \
  -H 'Authorization: Basic YXBpOg==' \
  -d 'SELECT getAhrefsURLFromUnparsed(src_unparsed) AS found_url, ifNull(toUnixTimestamp(download_stamp), 0) AS crawl_time, ifNull(toUnixTimestamp(props_url_first_seen), 0) AS first_indexed_time, download_http_code AS http_code, src_unparsed AS src_unparsed, src_root_hash AS src_root_hash, history_drop_reason AS history_drop_reason, meta_title AS meta_title, meta_descriptions AS meta_descriptions, attrs_boilerpipe_text AS attrs_boilerpipe_text, attrs_markdown AS attrs_markdown, attrs_readable_markdown AS attrs_readable_markdown, meta_canonical AS meta_canonical FROM crawler3.page_info_local FINAL PREWHERE (src_root_hash, src_unparsed) IN ((getAhrefsRootHashFromUnparsed(getAhrefsUnparsedNoserviceFromURL(\'https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions\')), getAhrefsUnparsedNoserviceFromURL(\'https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions\'))) FORMAT JSONEachRow'

Response:

{"found_url":"https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions","crawl_time":1775755146,"first_indexed_time":1410418860,"http_code":200,"src_unparsed":"org,mozilla!developer,\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions s443","src_root_hash":"7082249407640205653","history_drop_reason":null,"meta_title":"Functions - JavaScript | MDN","meta_descriptions":["Generally speaking, a function is a \"subprogram\" that can be called by code external (or internal, in the case of recursion) to the function. Like the program itself, a function is composed of a sequence of statements called the function body. Values can be passed to a function as parameters, and the function will return a value."],"attrs_boilerpipe_text":"Description\nFunction values are typically instances of\nFunction\n. See\nFunction\nfor information on properties and methods of\nFunction\nobjects. Callable values cause\ntypeof\nto return\n\"function\"\ninstead of\n\"object\"\n.\nNote:\nNot all callable values are\ninstanceof Function\n. For example, the\nFunction.prototype\nobject is callable but not an instance of\nFunction\n. You can also manually set the\nprototype chain\nof your function so it no longer inherits from\nFunction.prototype\n. However, such cases are extremely rare.\nReturn value\nBy default, if a function's execution doesn't end at a\nreturn\nstatement, or if the\nreturn\nkeyword doesn't have an expression after it, then the return value is\nundefined\n. The\nreturn\nstatement allows you to return an arbitrary value from the function. One function call can only return one value, but you can simulate the effect of returning multiple values by returning an object or array and\ndestructuring\nthe result.\nNote:\nConstructors called with\nnew\nhave a different set of logic to determine their return values.\nPassing arguments\nParameters and arguments\nhave slightly different meanings, but in MDN web docs, we often use them interchangeably. For a quick reference:\nIn this example, the\nnum\nvariable is called the function's\nparameter\n: it's declared in the parenthesis-enclosed list of the function's definition. The function expects the\nnum\nparameter to be a number — although this is not enforceable in JavaScript without writing runtime validation code. In the\nformatNumber(2)\ncall, the number\n2\nis the function's\nargument\n: it's the value that is actually passed to the function in the function call. The argument value can be accessed inside the function body through the corresponding parameter name or the\narguments\nobject.\nArguments are always\npassed by value\nand never\npassed by reference\n. This means that if a function reassigns a parameter, the value won't change outside the function. More precisely, object arguments are\npassed by sharing\n, which means if the object's properties are mutated, the change will impact the outside of the function. For example:\nThe\nthis\nkeyword refers to the object that the function is accessed on — it does not refer to the currently executing function, so you must refer to the function value by name, even within the function body.\nDefining functions\nBroadly speaking, JavaScript has four kinds of functions:\nRegular function: can return anything; always runs to completion after invocation\nGenerator function: returns a\nGenerator\nobject; can be paused and resumed with the\nyield\noperator\nAsync function: returns a\nPromise\n; can be paused and resumed with the\nawait\noperator\nAsync generator function: returns an\nAsyncGenerator\nobject; both the\nawait\nand\nyield\noperators can be used\nFor every kind of function, there are multiple ways to define it:\nDeclaration\nfunction\n,\nfunction*\n,\nasync function\n,\nasync function*\nExpression\nfunction\n,\nfunction*\n,\nasync function\n,\nasync function*\nConstructor\nFunction()\n,\nGeneratorFunction()\n,\nAsyncFunction()\n,\nAsyncGeneratorFunction()\nIn addition, there are special syntaxes for defining\narrow functions\nand\nmethods\n, which provide more precise semantics for their usage.\nClasses\nare conceptually not functions (because they throw an error when called without\nnew\n), but they also inherit from\nFunction.prototype\nand have\ntypeof MyClass === \"function\"\n.\nAll syntaxes do approximately the same thing, but there are some subtle behavior differences.\nThe\nFunction()\nconstructor,\nfunction\nexpression, and\nfunction\ndeclaration syntaxes create full-fledged function objects, which can be constructed with\nnew\n. However, arrow functions and methods cannot be constructed. Async functions, generator functions, and async generator functions are not constructible regardless of syntax.\nThe\nfunction\ndeclaration creates functions that are\nhoisted\n. Other syntaxes do not hoist the function and the function value is only visible after the definition.\nThe arrow function and\nFunction()\nconstructor always create\nanonymous\nfunctions, which means they can't easily call themselves recursively. One way to call an arrow function recursively is by assigning it to a variable.\nThe arrow function syntax does not have access to\narguments\nor\nthis\n.\nThe\nFunction()\nconstructor cannot access any local variables — it only has access to the global scope.\nThe\nFunction()\nconstructor causes runtime compilation and is often slower than other syntaxes.\nFor\nfunction\nexpressions, there is a distinction between the function name and the variable the function is assigned to. The function name cannot be changed, while the variable the function is assigned to can be reassigned. The function name can be different from the variable the function is assigned to — they have no relation to each other. The function name can be used only within the function's body. Attempting to use it outside the function's body results in an error (or gets another value, if the same name is declared elsewhere). For example:\nOn the other hand, the variable the function is assigned to is limited only by its scope, which is guaranteed to include the scope in which the function is declared.\nA function declaration also creates a variable with the same name as the function name. Thus, unlike those defined by function expressions, functions defined by function declarations can be accessed by their name in the scope they were defined in, as well as in their own body.\nA function defined by\nnew Function\nwill dynamically have its source assembled, which is observable when you serialize it. For example,\nconsole.log(new Function().toString())\ngives:\nThis is the actual source used to compile the function. However, although the\nFunction()\nconstructor will create the function with name\nanonymous\n, this name is not added to the scope of the body. The body only ever has access to global variables. For example, the following would result in an error:\nA function defined by a function expression or by a function declaration inherits the current scope. That is, the function forms a closure. On the other hand, a function defined by a\nFunction\nconstructor does not inherit any scope other than the global scope (which all functions inherit).\nFunctions defined by function expressions and function declarations are parsed only once, while a function defined by the\nFunction\nconstructor parses the string passed to it each and every time the constructor is called. Although a function expression creates a closure every time, the function body is not reparsed, so function expressions are still faster than\nnew Function(...)\n. Therefore the\nFunction\nconstructor should generally be avoided whenever possible.\nA function declaration may be unintentionally turned into a function expression when it appears in an expression context.\nOn the other hand, a function expression may also be turned into a function declaration. An\nexpression statement\ncannot begin with the\nfunction\nor\nasync function\nkeywords, which is a common mistake when implementing\nIIFEs\n(Immediately Invoked Function Expressions).\nInstead, start the expression statement with something else, so that the\nfunction\nkeyword unambiguously starts a function expression. Common options include\ngrouping\nand using\nvoid\n.\nFunction parameters\nEach function parameter is a simple identifier that you can access in the local scope.\nThere are three special parameter syntaxes:\nDefault parameters\nallow formal parameters to be initialized with default values if no value or\nundefined\nis passed.\nThe\nrest parameter\nallows representing an indefinite number of arguments as an array.\nDestructuring\nallows unpacking elements from arrays, or properties from objects, into distinct variables.\nThere are some consequences if one of the above non-simple parameter syntaxes is used:\nYou cannot apply\n\"use strict\"\nto the function body — this causes a\nsyntax error\n.\nEven if the function is not in\nstrict mode\n, certain strict mode function features apply, including that the\narguments\nobject stops syncing with the named parameters,\narguments.callee\nthrows an error when accessed, and duplicate parameter names are not allowed.\nThe arguments object\nYou can refer to a function's arguments within the function by using the\narguments\nobject.\narguments\nAn array-like object containing the arguments passed to the currently executing function.\narguments.callee\nThe currently executing function.\narguments.length\nThe number of arguments passed to the function.\nGetter and setter functions\nYou can define accessor properties on any standard built-in object or user-defined object that supports the addition of new properties. Within\nobject literals\nand\nclasses\n, you can use special syntaxes to define the getter and setter of an accessor property.\nget\nBinds an object property to a function that will be called when that property is looked up.\nset\nBinds an object property to a function to be called when there is an attempt to set that property.\nNote that these syntaxes create an\nobject property\n, not a\nmethod\n. The getter and setter functions themselves can only be accessed using reflective APIs such as\nObject.getOwnPropertyDescriptor()\n.\nBlock-level functions\nIn\nstrict mode\n, functions inside blocks are scoped to that block. Prior to ES2015, block-level functions were forbidden in strict mode.\nBlock-level functions in non-strict code\nIn a word:\nDon't.\nIn non-strict code, function declarations inside blocks behave strangely. For example:\nThe semantics of this in strict mode are well-specified —\nzero\nonly ever exists within that scope of the\nif\nblock. If\nshouldDefineZero\nis false, then\nzero\nshould never be defined, since the block never executes. However, historically, this was left unspecified, so different browsers implemented it differently in non-strict mode. For more information, see the\nfunction\ndeclaration\nreference.\nA safer way to define functions conditionally is to assign a function expression to a variable:\nExamples\nReturning a formatted number\nThe following function returns a string containing the formatted representation of a number padded with leading zeros.\nThe following statements call the\npadZeros\nfunction.\nDetermining whether a function exists\nYou can determine whether a function exists by using the\ntypeof\noperator. In the following example, a test is performed to determine if the\nwindow\nobject has a property called\nnoFunc\nthat is a function. If so, it is used; otherwise, some other action is taken.\nNote that in the\nif\ntest, a reference to\nnoFunc\nis used — there are no parentheses\n()\nafter the function name so the actual function is not called.\nSpecifications\nSpecification\nECMAScript® 2027 Language Specification\n# sec-function-definitions\nBrowser compatibility\nSee also\nFunctions\nguide\nClasses\nfunction\nfunction\nexpression\nFunction","attrs_markdown":"- [Skip to main content](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#content)\n- [Skip to search](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#search)\n\nHTML\n\n[HTML: Markup language](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/HTML)\n\nHTML reference\n\n- [Elements](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/HTML\/Reference\/Elements)\n- [Global attributes](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/HTML\/Reference\/Global_attributes)\n- [Attributes](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/HTML\/Reference\/Attributes)\n- [See all…](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/HTML\/Reference \"See all HTML references\")\n\nHTML guides\n\n- [Responsive images](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/HTML\/Guides\/Responsive_images)\n- [HTML cheatsheet](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/HTML\/Guides\/Cheatsheet)\n- [Date & time formats](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/HTML\/Guides\/Date_and_time_formats)\n- [See all…](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/HTML\/Guides \"See all HTML guides\")\n\nMarkup languages\n\n- [SVG](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/SVG)\n- [MathML](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/MathML)\n- [XML](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/XML)\n\nCSS\n\n[CSS: Styling language](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/CSS)\n\nCSS reference\n\n- [Properties](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/CSS\/Reference\/Properties)\n- [Selectors](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/CSS\/Reference\/Selectors)\n- [At-rules](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/CSS\/Reference\/At-rules)\n- [Values](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/CSS\/Reference\/Values)\n- [See all…](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/CSS\/Reference \"See all CSS references\")\n\nCSS guides\n\n- [Box model](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/CSS\/Guides\/Box_model\/Introduction)\n- [Animations](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/CSS\/Guides\/Animations\/Using)\n- [Flexbox](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/CSS\/Guides\/Flexible_box_layout\/Basic_concepts)\n- [Colors](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/CSS\/Guides\/Colors\/Applying_color)\n- [See all…](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/CSS\/Guides \"See all CSS guides\")\n\nLayout cookbook\n\n- [Column layouts](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/CSS\/How_to\/Layout_cookbook\/Column_layouts)\n- [Centering an element](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/CSS\/How_to\/Layout_cookbook\/Center_an_element)\n- [Card component](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/CSS\/How_to\/Layout_cookbook\/Card)\n- [See all…](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/CSS\/How_to\/Layout_cookbook)\n\nJavaScriptJS\n\n[JavaScript: Scripting language](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript)\n\nJS reference\n\n- [Standard built-in objects](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects)\n- [Expressions & operators](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators)\n- [Statements & declarations](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements)\n- [Functions](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions)\n- [See all…](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference \"See all JavaScript references\")\n\nJS guides\n\n- [Control flow & error handing](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Control_flow_and_error_handling)\n- [Loops and iteration](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Loops_and_iteration)\n- [Working with objects](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Working_with_objects)\n- [Using classes](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Using_classes)\n- [See all…](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide \"See all JavaScript guides\")\n\nWeb APIs\n\n[Web APIs: Programming interfaces](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/API)\n\nWeb API reference\n\n- [File system API](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/API\/File_System_API)\n- [Fetch API](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/API\/Fetch_API)\n- [Geolocation API](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/API\/Geolocation_API)\n- [HTML DOM API](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/API\/HTML_DOM_API)\n- [Push API](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/API\/Push_API)\n- [Service worker API](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/API\/Service_Worker_API)\n- [See all…](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/API \"See all Web API guides\")\n\nWeb API guides\n\n- [Using the Web animation API](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/API\/Web_Animations_API\/Using_the_Web_Animations_API)\n- [Using the Fetch API](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/API\/Fetch_API\/Using_Fetch)\n- [Working with the History API](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/API\/History_API\/Working_with_the_History_API)\n- [Using the Web speech API](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/API\/Web_Speech_API\/Using_the_Web_Speech_API)\n- [Using web workers](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/API\/Web_Workers_API\/Using_web_workers)\n\nAll\n\n[All web technology](https:\/\/developer.mozilla.org\/en-US\/docs\/Web)\n\nTechnologies\n\n- [Accessibility](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/Accessibility)\n- [HTTP](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/HTTP)\n- [URI](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/URI)\n- [Web extensions](https:\/\/developer.mozilla.org\/en-US\/docs\/Mozilla\/Add-ons\/WebExtensions)\n- [WebAssembly](https:\/\/developer.mozilla.org\/en-US\/docs\/WebAssembly)\n- [WebDriver](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/WebDriver)\n- [See all…](https:\/\/developer.mozilla.org\/en-US\/docs\/Web \"See all web technology references\")\n\nTopics\n\n- [Media](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/Media)\n- [Performance](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/Performance)\n- [Privacy](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/Privacy)\n- [Security](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/Security)\n- [Progressive web apps](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/Progressive_web_apps)\n\nLearn\n\n[Learn web development](https:\/\/developer.mozilla.org\/en-US\/docs\/Learn_web_development)\n\nFrontend developer course\n\n- [Getting started modules](https:\/\/developer.mozilla.org\/en-US\/docs\/Learn_web_development\/Getting_started)\n- [Core modules](https:\/\/developer.mozilla.org\/en-US\/docs\/Learn_web_development\/Core)\n- [MDN Curriculum](https:\/\/developer.mozilla.org\/en-US\/curriculum\/)\n- [Check out the video course from Scrimba, our partner](https:\/\/scrimba.com\/frontend-path-c0j?via=mdn-learn-navbar)\n\nLearn HTML\n\n- [Structuring content with HTML module](https:\/\/developer.mozilla.org\/en-US\/docs\/Learn_web_development\/Core\/Structuring_content)\n\nLearn CSS\n\n- [CSS styling basics module](https:\/\/developer.mozilla.org\/en-US\/docs\/Learn_web_development\/Core\/Styling_basics)\n- [CSS layout module](https:\/\/developer.mozilla.org\/en-US\/docs\/Learn_web_development\/Core\/CSS_layout)\n\nLearn JavaScript\n\n- [Dynamic scripting with JavaScript module](https:\/\/developer.mozilla.org\/en-US\/docs\/Learn_web_development\/Core\/Scripting)\n\nTools\n\nDiscover our tools\n\n- [Playground](https:\/\/developer.mozilla.org\/en-US\/play)\n- [HTTP Observatory](https:\/\/developer.mozilla.org\/en-US\/observatory)\n\n- [Border-image generator](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/CSS\/Guides\/Backgrounds_and_borders\/Border-image_generator)\n- [Border-radius generator](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/CSS\/Guides\/Backgrounds_and_borders\/Border-radius_generator)\n- [Box-shadow generator](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/CSS\/Guides\/Backgrounds_and_borders\/Box-shadow_generator)\n- [Color format converter](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/CSS\/Guides\/Colors\/Color_format_converter)\n- [Color mixer](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/CSS\/Guides\/Colors\/Color_mixer)\n- [Shape generator](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/CSS\/Guides\/Shapes\/Shape_generator)\n\nAbout\n\nGet to know MDN better\n\n- [About MDN](https:\/\/developer.mozilla.org\/en-US\/about)\n- [Advertise with us](https:\/\/developer.mozilla.org\/en-US\/advertising)\n\n- [Community](https:\/\/developer.mozilla.org\/en-US\/community)\n- [MDN on GitHub](https:\/\/github.com\/mdn)\n\n[Blog](https:\/\/developer.mozilla.org\/en-US\/blog\/)\n\n1. [Web](https:\/\/developer.mozilla.org\/en-US\/docs\/Web)\n2. [JavaScript](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript)\n3. [Reference](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference)\n4. [Functions](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions)\n\n# Functions\nBaseline  Widely available  \\*\n\nThis feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.\n\n\\* Some parts of this feature may have varying levels of support.\n\n- [Learn more](https:\/\/developer.mozilla.org\/en-US\/docs\/Glossary\/Baseline\/Compatibility)\n- [See full compatibility](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#browser_compatibility)\n- [Report feedback](https:\/\/survey.alchemer.com\/s3\/7634825\/MDN-baseline-feedback?page=%2Fen-US%2Fdocs%2FWeb%2FJavaScript%2FReference%2FFunctions&level=high)\n\nGenerally speaking, a function is a \"subprogram\" that can be *called* by code external (or internal, in the case of recursion) to the function. Like the program itself, a function is composed of a sequence of statements called the *function body*. Values can be *passed* to a function as parameters, and the function will *return* a value.\n\nIn JavaScript, functions are [first-class objects](https:\/\/developer.mozilla.org\/en-US\/docs\/Glossary\/First-class_Function), because they can be passed to other functions, returned from functions, and assigned to variables and properties. They can also have properties and methods just like any other object. What distinguishes them from other objects is that functions can be called.\n\nFor more examples and explanations, see the [JavaScript guide about functions](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Functions).\n\n## In this article\n- [Description](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#description)\n- [Examples](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#examples)\n- [Specifications](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#specifications)\n- [Browser compatibility](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#browser_compatibility)\n- [See also](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#see_also)\n\n## [Description](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#description)\nFunction values are typically instances of [`Function`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Function). See [`Function`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Function) for information on properties and methods of `Function` objects. Callable values cause [`typeof`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/typeof) to return `\"function\"` instead of `\"object\"`.\n\n**Note:** Not all callable values are `instanceof Function`. For example, the `Function.prototype` object is callable but not an instance of `Function`. You can also manually set the [prototype chain](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Inheritance_and_the_prototype_chain) of your function so it no longer inherits from `Function.prototype`. However, such cases are extremely rare.\n\n### [Return value](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#return_value)\nBy default, if a function's execution doesn't end at a [`return`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/return) statement, or if the `return` keyword doesn't have an expression after it, then the return value is [`undefined`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/undefined). The `return` statement allows you to return an arbitrary value from the function. One function call can only return one value, but you can simulate the effect of returning multiple values by returning an object or array and [destructuring](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Destructuring) the result.\n\n**Note:** Constructors called with [`new`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/new) have a different set of logic to determine their return values.\n\n### [Passing arguments](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#passing_arguments)\n[Parameters and arguments](https:\/\/en.wikipedia.org\/wiki\/Parameter_\$computer_programming\$#Parameters_and_arguments) have slightly different meanings, but in MDN web docs, we often use them interchangeably. For a quick reference:\n\nIn this example, the `num` variable is called the function's *parameter*: it's declared in the parenthesis-enclosed list of the function's definition. The function expects the `num` parameter to be a number — although this is not enforceable in JavaScript without writing runtime validation code. In the `formatNumber(2)` call, the number `2` is the function's *argument*: it's the value that is actually passed to the function in the function call. The argument value can be accessed inside the function body through the corresponding parameter name or the [`arguments`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/arguments) object.\n\nArguments are always [*passed by value*](https:\/\/en.wikipedia.org\/wiki\/Evaluation_strategy#Call_by_reference) and never *passed by reference*. This means that if a function reassigns a parameter, the value won't change outside the function. More precisely, object arguments are [*passed by sharing*](https:\/\/en.wikipedia.org\/wiki\/Evaluation_strategy#Call_by_sharing), which means if the object's properties are mutated, the change will impact the outside of the function. For example:\n\nThe [`this`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/this) keyword refers to the object that the function is accessed on — it does not refer to the currently executing function, so you must refer to the function value by name, even within the function body.\n\n### [Defining functions](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#defining_functions)\nBroadly speaking, JavaScript has four kinds of functions:\n\n- Regular function: can return anything; always runs to completion after invocation\n- Generator function: returns a [`Generator`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Generator) object; can be paused and resumed with the [`yield`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/yield) operator\n- Async function: returns a [`Promise`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Promise); can be paused and resumed with the [`await`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/await) operator\n- Async generator function: returns an [`AsyncGenerator`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/AsyncGenerator) object; both the `await` and `yield` operators can be used\n\nFor every kind of function, there are multiple ways to define it:\n\n[Declaration](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#declaration)\n\n[`function`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/function), [`function*`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/function*), [`async function`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/async_function), [`async function*`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/async_function*)\n\n[Expression](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#expression)\n\n[`function`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/function), [`function*`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/function*), [`async function`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/async_function), [`async function*`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/async_function*)\n\n[Constructor](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#constructor)\n\n[`Function()`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Function\/Function), [`GeneratorFunction()`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/GeneratorFunction\/GeneratorFunction), [`AsyncFunction()`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/AsyncFunction\/AsyncFunction), [`AsyncGeneratorFunction()`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/AsyncGeneratorFunction\/AsyncGeneratorFunction)\n\nIn addition, there are special syntaxes for defining [arrow functions](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/Arrow_functions) and [methods](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/Method_definitions), which provide more precise semantics for their usage. [Classes](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Classes) are conceptually not functions (because they throw an error when called without `new`), but they also inherit from `Function.prototype` and have `typeof MyClass === \"function\"`.\n\nAll syntaxes do approximately the same thing, but there are some subtle behavior differences.\n\n- The `Function()` constructor, `function` expression, and `function` declaration syntaxes create full-fledged function objects, which can be constructed with [`new`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/new). However, arrow functions and methods cannot be constructed. Async functions, generator functions, and async generator functions are not constructible regardless of syntax.\n- The `function` declaration creates functions that are [*hoisted*](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Functions#function_hoisting). Other syntaxes do not hoist the function and the function value is only visible after the definition.\n- The arrow function and `Function()` constructor always create *anonymous* functions, which means they can't easily call themselves recursively. One way to call an arrow function recursively is by assigning it to a variable.\n- The arrow function syntax does not have access to `arguments` or `this`.\n- The `Function()` constructor cannot access any local variables — it only has access to the global scope.\n- The `Function()` constructor causes runtime compilation and is often slower than other syntaxes.\n\nFor `function` expressions, there is a distinction between the function name and the variable the function is assigned to. The function name cannot be changed, while the variable the function is assigned to can be reassigned. The function name can be different from the variable the function is assigned to — they have no relation to each other. The function name can be used only within the function's body. Attempting to use it outside the function's body results in an error (or gets another value, if the same name is declared elsewhere). For example:\n\nOn the other hand, the variable the function is assigned to is limited only by its scope, which is guaranteed to include the scope in which the function is declared.\n\nA function declaration also creates a variable with the same name as the function name. Thus, unlike those defined by function expressions, functions defined by function declarations can be accessed by their name in the scope they were defined in, as well as in their own body.\n\nA function defined by `new Function` will dynamically have its source assembled, which is observable when you serialize it. For example, `console.log(new Function().toString())` gives:\n\nThis is the actual source used to compile the function. However, although the `Function()` constructor will create the function with name `anonymous`, this name is not added to the scope of the body. The body only ever has access to global variables. For example, the following would result in an error:\n\nA function defined by a function expression or by a function declaration inherits the current scope. That is, the function forms a closure. On the other hand, a function defined by a `Function` constructor does not inherit any scope other than the global scope (which all functions inherit).\n\nFunctions defined by function expressions and function declarations are parsed only once, while a function defined by the `Function` constructor parses the string passed to it each and every time the constructor is called. Although a function expression creates a closure every time, the function body is not reparsed, so function expressions are still faster than `new Function(...)`. Therefore the `Function` constructor should generally be avoided whenever possible.\n\nA function declaration may be unintentionally turned into a function expression when it appears in an expression context.\n\nOn the other hand, a function expression may also be turned into a function declaration. An [expression statement](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/Expression_statement) cannot begin with the `function` or `async function` keywords, which is a common mistake when implementing [IIFEs](https:\/\/developer.mozilla.org\/en-US\/docs\/Glossary\/IIFE) (Immediately Invoked Function Expressions).\n\nInstead, start the expression statement with something else, so that the `function` keyword unambiguously starts a function expression. Common options include [grouping](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Grouping) and using [`void`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/void).\n\n### [Function parameters](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#function_parameters)\nEach function parameter is a simple identifier that you can access in the local scope.\n\nThere are three special parameter syntaxes:\n\n- [*Default parameters*](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/Default_parameters) allow formal parameters to be initialized with default values if no value or `undefined` is passed.\n- The [*rest parameter*](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/rest_parameters) allows representing an indefinite number of arguments as an array.\n- [*Destructuring*](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Destructuring) allows unpacking elements from arrays, or properties from objects, into distinct variables.\n\nThere are some consequences if one of the above non-simple parameter syntaxes is used:\n\n- You cannot apply `\"use strict\"` to the function body — this causes a [syntax error](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Strict_non_simple_params).\n- Even if the function is not in [strict mode](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Strict_mode), certain strict mode function features apply, including that the [`arguments`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/arguments) object stops syncing with the named parameters, [`arguments.callee`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/arguments\/callee) throws an error when accessed, and duplicate parameter names are not allowed.\n\n### [The arguments object](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#the_arguments_object)\nYou can refer to a function's arguments within the function by using the [`arguments`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/arguments) object.\n\n[`arguments`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/arguments)\n\nAn array-like object containing the arguments passed to the currently executing function.\n\n[`arguments.callee`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/arguments\/callee)\n\nThe currently executing function.\n\n[`arguments.length`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/arguments\/length)\n\nThe number of arguments passed to the function.\n\n### [Getter and setter functions](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#getter_and_setter_functions)\nYou can define accessor properties on any standard built-in object or user-defined object that supports the addition of new properties. Within [object literals](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Object_initializer) and [classes](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Classes), you can use special syntaxes to define the getter and setter of an accessor property.\n\n[get](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/get)\n\nBinds an object property to a function that will be called when that property is looked up.\n\n[set](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/set)\n\nBinds an object property to a function to be called when there is an attempt to set that property.\n\nNote that these syntaxes create an *object property*, not a *method*. The getter and setter functions themselves can only be accessed using reflective APIs such as [`Object.getOwnPropertyDescriptor()`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Object\/getOwnPropertyDescriptor).\n\n### [Block-level functions](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#block-level_functions)\nIn [strict mode](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Strict_mode), functions inside blocks are scoped to that block. Prior to ES2015, block-level functions were forbidden in strict mode.\n\n### [Block-level functions in non-strict code](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#block-level_functions_in_non-strict_code)\nIn a word: **Don't.**\n\nIn non-strict code, function declarations inside blocks behave strangely. For example:\n\nThe semantics of this in strict mode are well-specified — `zero` only ever exists within that scope of the `if` block. If `shouldDefineZero` is false, then `zero` should never be defined, since the block never executes. However, historically, this was left unspecified, so different browsers implemented it differently in non-strict mode. For more information, see the [`function` declaration](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/function#block-level_function_declaration) reference.\n\nA safer way to define functions conditionally is to assign a function expression to a variable:\n## [Examples](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#examples)\n### [Returning a formatted number](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#returning_a_formatted_number)\nThe following function returns a string containing the formatted representation of a number padded with leading zeros.\n\nThe following statements call the `padZeros` function.\n\n### [Determining whether a function exists](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#determining_whether_a_function_exists)\nYou can determine whether a function exists by using the [`typeof`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/typeof) operator. In the following example, a test is performed to determine if the `window` object has a property called `noFunc` that is a function. If so, it is used; otherwise, some other action is taken.\n\nNote that in the `if` test, a reference to `noFunc` is used — there are no parentheses `()` after the function name so the actual function is not called.\n\n## [Specifications](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#specifications)\n| Specification |\n|---|\n| [ECMAScript® 2027 Language Specification \\# sec-function-definitions](https:\/\/tc39.es\/ecma262\/multipage\/ecmascript-language-functions-and-classes.html#sec-function-definitions) |\n## [Browser compatibility](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#browser_compatibility)\n## [See also](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#see_also)\n- [Functions](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Functions) guide\n- [Classes](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Classes)\n- [`function`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/function)\n- [`function` expression](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/function)\n- [`Function`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Function)\n\n## Help improve MDN\n[Learn how to contribute](https:\/\/developer.mozilla.org\/en-US\/docs\/MDN\/Community\/Getting_started)\n\nThis page was last modified on Jul 8, 2025 by [MDN contributors](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/contributors.txt).\n\n[View this page on GitHub](https:\/\/github.com\/mdn\/content\/blob\/main\/files\/en-us\/web\/javascript\/reference\/functions\/index.md?plain=1 \"Folder: en-us\/web\/javascript\/reference\/functions (Opens in a new tab)\") • [Report a problem with this content](https:\/\/github.com\/mdn\/content\/issues\/new?template=page-report.yml&mdn-url=https%3A%2F%2Fdeveloper.mozilla.org%2Fen-US%2Fdocs%2FWeb%2FJavaScript%2FReference%2FFunctions&metadata=%3C%21--+Do+not+make+changes+below+this+line+--%3E%0A%3Cdetails%3E%0A%3Csummary%3EPage+report+details%3C%2Fsummary%3E%0A%0A*+Folder%3A+%60en-us%2Fweb%2Fjavascript%2Freference%2Ffunctions%60%0A*+MDN+URL%3A+https%3A%2F%2Fdeveloper.mozilla.org%2Fen-US%2Fdocs%2FWeb%2FJavaScript%2FReference%2FFunctions%0A*+GitHub+URL%3A+https%3A%2F%2Fgithub.com%2Fmdn%2Fcontent%2Fblob%2Fmain%2Ffiles%2Fen-us%2Fweb%2Fjavascript%2Freference%2Ffunctions%2Findex.md%0A*+Last+commit%3A+https%3A%2F%2Fgithub.com%2Fmdn%2Fcontent%2Fcommit%2Ffad67be4431d8e6c2a89ac880735233aa76c41d4%0A*+Document+last+modified%3A+2025-07-08T13%3A18%3A45.000Z%0A%0A%3C%2Fdetails%3E \"This will take you to GitHub to file a new issue.\")\n\n1. [JavaScript](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript)\n2. Tutorials and guides\n3. [JavaScript Guide](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide)\n   \n   1. [Introduction](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Introduction)\n   2. [Grammar and types](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Grammar_and_types)\n   3. [Control flow and error handling](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Control_flow_and_error_handling)\n   4. [Loops and iteration](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Loops_and_iteration)\n   5. [Functions](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Functions)\n   6. [Expressions and operators](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Expressions_and_operators)\n   7. [Numbers and strings](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Numbers_and_strings)\n   8. [Representing dates & times](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Representing_dates_times)\n   9. [Regular expressions](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Regular_expressions)\n   10. [Indexed collections](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Indexed_collections)\n   11. [Keyed collections](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Keyed_collections)\n   12. [Working with objects](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Working_with_objects)\n   13. [Using classes](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Using_classes)\n   14. [Using promises](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Using_promises)\n   15. [JavaScript typed arrays](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Typed_arrays)\n   16. [Iterators and generators](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Iterators_and_generators)\n   17. [Resource management](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Resource_management)\n   18. [Internationalization](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Internationalization)\n   19. [JavaScript modules](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Modules)\n4. Intermediate\n   \n   1. [Language overview](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Language_overview)\n   2. [JavaScript data structures](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Data_structures)\n   3. [Equality comparisons and sameness](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Equality_comparisons_and_sameness)\n   4. [Enumerability and ownership of properties](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Enumerability_and_ownership_of_properties)\n   5. [Closures](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Closures)\n5. Advanced\n   \n   1. [Inheritance and the prototype chain](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Inheritance_and_the_prototype_chain)\n   2. [Meta programming](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Meta_programming)\n   3. [Memory Management](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Memory_management)\n6. [References](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference)\n7. [Built-in objects](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects)\n   \n   1. [AggregateError](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/AggregateError)\n   2. [Array](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Array)\n   3. [ArrayBuffer](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/ArrayBuffer)\n   4. [AsyncDisposableStack](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/AsyncDisposableStack)\n   5. [AsyncFunction](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/AsyncFunction)\n   6. [AsyncGenerator](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/AsyncGenerator)\n   7. [AsyncGeneratorFunction](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/AsyncGeneratorFunction)\n   8. [AsyncIterator](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/AsyncIterator)\n   9. [Atomics](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Atomics)\n   10. [BigInt](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/BigInt)\n   11. [BigInt64Array](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/BigInt64Array)\n   12. [BigUint64Array](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/BigUint64Array)\n   13. [Boolean](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Boolean)\n   14. [DataView](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/DataView)\n   15. [Date](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Date)\n   16. [decodeURI()](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/decodeURI)\n   17. [decodeURIComponent()](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/decodeURIComponent)\n   18. [DisposableStack](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/DisposableStack)\n   19. [encodeURI()](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/encodeURI)\n   20. [encodeURIComponent()](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/encodeURIComponent)\n   21. [Error](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Error)\n   22. [escape()](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/escape)\n       Deprecated\n   23. [eval()](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/eval)\n   24. [EvalError](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/EvalError)\n   25. [FinalizationRegistry](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/FinalizationRegistry)\n   26. [Float16Array](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Float16Array)\n   27. [Float32Array](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Float32Array)\n   28. [Float64Array](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Float64Array)\n   29. [Function](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Function)\n   30. [Generator](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Generator)\n   31. [GeneratorFunction](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/GeneratorFunction)\n   32. [globalThis](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/globalThis)\n   33. [Infinity](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Infinity)\n   34. [Int8Array](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Int8Array)\n   35. [Int16Array](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Int16Array)\n   36. [Int32Array](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Int32Array)\n   37. [InternalError](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/InternalError)\n       Non-standard\n   38. [Intl](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Intl)\n   39. [isFinite()](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/isFinite)\n   40. [isNaN()](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/isNaN)\n   41. [Iterator](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Iterator)\n   42. [JSON](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/JSON)\n   43. [Map](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Map)\n   44. [Math](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Math)\n   45. [NaN](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/NaN)\n   46. [Number](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Number)\n   47. [Object](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Object)\n   48. [parseFloat()](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/parseFloat)\n   49. [parseInt()](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/parseInt)\n   50. [Promise](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Promise)\n   51. [Proxy](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Proxy)\n   52. [RangeError](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/RangeError)\n   53. [ReferenceError](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/ReferenceError)\n   54. [Reflect](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Reflect)\n   55. [RegExp](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/RegExp)\n   56. [Set](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Set)\n   57. [SharedArrayBuffer](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/SharedArrayBuffer)\n   58. [String](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/String)\n   59. [SuppressedError](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/SuppressedError)\n   60. [Symbol](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Symbol)\n   61. [SyntaxError](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/SyntaxError)\n   62. [Temporal](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Temporal)\n   63. [TypedArray](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/TypedArray)\n   64. [TypeError](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/TypeError)\n   65. [Uint8Array](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Uint8Array)\n   66. [Uint8ClampedArray](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Uint8ClampedArray)\n   67. [Uint16Array](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Uint16Array)\n   68. [Uint32Array](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Uint32Array)\n   69. [undefined](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/undefined)\n   70. [unescape()](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/unescape)\n       Deprecated\n   71. [URIError](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/URIError)\n   72. [WeakMap](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/WeakMap)\n   73. [WeakRef](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/WeakRef)\n   74. [WeakSet](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/WeakSet)\n8. [Expressions & operators](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators)\n   \n   1. [Addition (+)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Addition)\n   2. [Addition assignment (+=)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Addition_assignment)\n   3. [Assignment (=)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Assignment)\n   4. [async function expression](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/async_function)\n   5. [async function\\* expression](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/async_function*)\n   6. [await](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/await)\n   7. [Bitwise AND (&)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Bitwise_AND)\n   8. [Bitwise AND assignment (&=)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Bitwise_AND_assignment)\n   9. [Bitwise NOT (~)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Bitwise_NOT)\n   10. [Bitwise OR (\\|)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Bitwise_OR)\n   11. [Bitwise OR assignment (\\|=)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Bitwise_OR_assignment)\n   12. [Bitwise XOR (^)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Bitwise_XOR)\n   13. [Bitwise XOR assignment (^=)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Bitwise_XOR_assignment)\n   14. [class expression](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/class)\n   15. [Comma operator (,)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Comma_operator)\n   16. [Conditional (ternary) operator](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Conditional_operator)\n   17. [Decrement (--)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Decrement)\n   18. [delete](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/delete)\n   19. [Destructuring](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Destructuring)\n   20. [Division (\/)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Division)\n   21. [Division assignment (\/=)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Division_assignment)\n   22. [Equality (==)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Equality)\n   23. [Exponentiation (\\*\\*)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Exponentiation)\n   24. [Exponentiation assignment (\\*\\*=)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Exponentiation_assignment)\n   25. [function expression](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/function)\n   26. [function\\* expression](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/function*)\n   27. [Greater than (\\>)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Greater_than)\n   28. [Greater than or equal (\\>=)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Greater_than_or_equal)\n   29. [Grouping operator ( )](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Grouping)\n   30. [import.meta](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/import.meta)\n       1. [import.meta.resolve()](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/import.meta\/resolve)\n   31. [import()](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/import)\n   32. [in](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/in)\n   33. [Increment (++)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Increment)\n   34. [Inequality (!=)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Inequality)\n   35. [instanceof](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/instanceof)\n   36. [Left shift (\\<\\<)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Left_shift)\n   37. [Left shift assignment (\\<\\<=)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Left_shift_assignment)\n   38. [Less than (\\<)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Less_than)\n   39. [Less than or equal (\\<=)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Less_than_or_equal)\n   40. [Logical AND (&&)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Logical_AND)\n   41. [Logical AND assignment (&&=)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Logical_AND_assignment)\n   42. [Logical NOT (!)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Logical_NOT)\n   43. [Logical OR (\\|\\|)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Logical_OR)\n   44. [Logical OR assignment (\\|\\|=)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Logical_OR_assignment)\n   45. [Multiplication (\\*)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Multiplication)\n   46. [Multiplication assignment (\\*=)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Multiplication_assignment)\n   47. [new](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/new)\n   48. [new.target](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/new.target)\n   49. [null](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/null)\n   50. [Nullish coalescing assignment (??=)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Nullish_coalescing_assignment)\n   51. [Nullish coalescing operator (??)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Nullish_coalescing)\n   52. [Object initializer](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Object_initializer)\n   53. [Operator precedence](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Operator_precedence)\n   54. [Optional chaining (?.)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Optional_chaining)\n   55. [Property accessors](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Property_accessors)\n   56. [Remainder (%)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Remainder)\n   57. [Remainder assignment (%=)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Remainder_assignment)\n   58. [Right shift (\\>\\>)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Right_shift)\n   59. [Right shift assignment (\\>\\>=)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Right_shift_assignment)\n   60. [Spread syntax (...)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Spread_syntax)\n   61. [Strict equality (===)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Strict_equality)\n   62. [Strict inequality (!==)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Strict_inequality)\n   63. [Subtraction (-)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Subtraction)\n   64. [Subtraction assignment (-=)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Subtraction_assignment)\n   65. [super](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/super)\n   66. [this](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/this)\n   67. [typeof](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/typeof)\n   68. [Unary negation (-)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Unary_negation)\n   69. [Unary plus (+)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Unary_plus)\n   70. [Unsigned right shift (\\>\\>\\>)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Unsigned_right_shift)\n   71. [Unsigned right shift assignment (\\>\\>\\>=)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Unsigned_right_shift_assignment)\n   72. [void operator](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/void)\n   73. [yield](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/yield)\n   74. [yield\\*](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/yield*)\n9. [Statements & declarations](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements)\n   \n   1. [async function](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/async_function)\n   2. [async function\\*](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/async_function*)\n   3. [await using](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/await_using)\n   4. [Block statement](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/block)\n   5. [break](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/break)\n   6. [class](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/class)\n   7. [const](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/const)\n   8. [continue](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/continue)\n   9. [debugger](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/debugger)\n   10. [do...while](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/do...while)\n   11. [Empty statement](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/Empty)\n   12. [export](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/export)\n   13. [Expression statement](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/Expression_statement)\n   14. [for](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/for)\n   15. [for await...of](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/for-await...of)\n   16. [for...in](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/for...in)\n   17. [for...of](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/for...of)\n   18. [function](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/function)\n   19. [function\\*](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/function*)\n   20. [if...else](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/if...else)\n   21. [import](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/import)\n       1. [Import attributes](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/import\/with)\n   22. [Labeled statement](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/label)\n   23. [let](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/let)\n   24. [return](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/return)\n   25. [switch](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/switch)\n   26. [throw](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/throw)\n   27. [try...catch](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/try...catch)\n   28. [using](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/using)\n   29. [var](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/var)\n   30. [while](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/while)\n   31. [with](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/with)\n       Deprecated\n10. *[Functions](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions)*\n    \n    1. [Arrow function expressions](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/Arrow_functions)\n    2. [Default parameters](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/Default_parameters)\n    3. [get](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/get)\n    4. [Method definitions](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/Method_definitions)\n    5. [Rest parameters](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/rest_parameters)\n    6. [set](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/set)\n    7. [The arguments object](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/arguments)\n       1. [\\[Symbol.iterator\\]()](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/arguments\/Symbol.iterator)\n       2. [callee](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/arguments\/callee)\n          Deprecated\n       3. [length](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/arguments\/length)\n11. [Classes](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Classes)\n    \n    1. [constructor](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Classes\/constructor)\n    2. [extends](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Classes\/extends)\n    3. [Private elements](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Classes\/Private_elements)\n    4. [Public class fields](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Classes\/Public_class_fields)\n    5. [static](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Classes\/static)\n    6. [Static initialization blocks](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Classes\/Static_initialization_blocks)\n12. [Regular expressions](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Regular_expressions)\n    \n    1. [Backreference: \\\\1, \\\\2](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Regular_expressions\/Backreference)\n    2. [Capturing group: (...)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Regular_expressions\/Capturing_group)\n    3. [Character class escape: \\\\d, \\\\D, \\\\w, \\\\W, \\\\s, \\\\S](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Regular_expressions\/Character_class_escape)\n    4. [Character class: \\[...\\], \\[^...\\]](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Regular_expressions\/Character_class)\n    5. [Character escape: \\\\n, \\\\u{...}](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Regular_expressions\/Character_escape)\n    6. [Disjunction: \\|](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Regular_expressions\/Disjunction)\n    7. [Input boundary assertion: ^, \\$](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Regular_expressions\/Input_boundary_assertion)\n    8. [Literal character: a, b](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Regular_expressions\/Literal_character)\n    9. [Lookahead assertion: (?=...), (?!...)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Regular_expressions\/Lookahead_assertion)\n    10. [Lookbehind assertion: (?\\<=...), (?\\<!...)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Regular_expressions\/Lookbehind_assertion)\n    11. [Modifier: (?ims-ims:...)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Regular_expressions\/Modifier)\n    12. [Named backreference: \\\\k\\<name\\>](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Regular_expressions\/Named_backreference)\n    13. [Named capturing group: (?\\<name\\>...)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Regular_expressions\/Named_capturing_group)\n    14. [Non-capturing group: (?:...)](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Regular_expressions\/Non-capturing_group)\n    15. [Quantifier: \\*, +, ?, {n}, {n,}, {n,m}](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Regular_expressions\/Quantifier)\n    16. [Unicode character class escape: \\\\p{...}, \\\\P{...}](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Regular_expressions\/Unicode_character_class_escape)\n    17. [Wildcard: .](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Regular_expressions\/Wildcard)\n    18. [Word boundary assertion: \\\\b, \\\\B](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Regular_expressions\/Word_boundary_assertion)\n13. [Errors](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors)\n    \n    1. [AggregateError: No Promise in Promise.any was resolved](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Promise_any_all_rejected)\n    2. [Error: Permission denied to access property \"x\"](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Property_access_denied)\n    3. [InternalError: too much recursion](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Too_much_recursion)\n    4. [RangeError: argument is not a valid code point](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Not_a_valid_code_point)\n    5. [RangeError: BigInt division by zero](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/BigInt_division_by_zero)\n    6. [RangeError: BigInt negative exponent](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/BigInt_negative_exponent)\n    7. [RangeError: form must be one of 'NFC', 'NFD', 'NFKC', or 'NFKD'](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Form_must_be_one_of)\n    8. [RangeError: invalid array length](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Invalid_array_length)\n    9. [RangeError: invalid date](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Invalid_date)\n    10. [RangeError: precision is out of range](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Precision_range)\n    11. [RangeError: radix must be an integer](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Bad_radix)\n    12. [RangeError: repeat count must be less than infinity](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Resulting_string_too_large)\n    13. [RangeError: repeat count must be non-negative](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Negative_repetition_count)\n    14. [RangeError: x can't be converted to BigInt because it isn't an integer](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Cant_be_converted_to_BigInt_because_it_isnt_an_integer)\n    15. [ReferenceError: \"x\" is not defined](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Not_defined)\n    16. [ReferenceError: assignment to undeclared variable \"x\"](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Undeclared_var)\n    17. [ReferenceError: can't access lexical declaration 'X' before initialization](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Cant_access_lexical_declaration_before_init)\n    18. [ReferenceError: must call super constructor before using 'this' in derived class constructor](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Super_not_called)\n    19. [ReferenceError: super() called twice in derived class constructor](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Super_called_twice)\n    20. [SyntaxError: 'arguments'\/'eval' can't be defined or assigned to in strict mode code](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Bad_strict_arguments_eval)\n    21. [SyntaxError: \"0\"-prefixed octal literals are deprecated](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Deprecated_octal_literal)\n    22. [SyntaxError: \"use strict\" not allowed in function with non-simple parameters](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Strict_non_simple_params)\n    23. [SyntaxError: \"x\" is a reserved identifier](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Reserved_identifier)\n    24. [SyntaxError: \\\\ at end of pattern](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Regex_backslash_at_end_of_pattern)\n    25. [SyntaxError: a declaration in the head of a for-of loop can't have an initializer](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Invalid_for-of_initializer)\n    26. [SyntaxError: applying the 'delete' operator to an unqualified name is deprecated](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Delete_in_strict_mode)\n    27. [SyntaxError: arguments is not valid in fields](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Arguments_not_allowed)\n    28. [SyntaxError: await is only valid in async functions, async generators and modules](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Bad_await)\n    29. [SyntaxError: await\/yield expression can't be used in parameter](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/await_yield_in_parameter)\n    30. [SyntaxError: cannot use ?? unparenthesized within \\|\\| and && expressions](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Cant_use_nullish_coalescing_unparenthesized)\n    31. [SyntaxError: character class escape cannot be used in class range in regular expression](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Regex_character_class_escape_in_class_range)\n    32. [SyntaxError: continue must be inside loop](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Bad_continue)\n    33. [SyntaxError: duplicate capture group name in regular expression](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Regex_duplicate_capture_group_name)\n    34. [SyntaxError: duplicate formal argument x](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Duplicate_parameter)\n    35. [SyntaxError: for-in loop head declarations may not have initializers](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Invalid_for-in_initializer)\n    36. [SyntaxError: function statement requires a name](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Unnamed_function_statement)\n    37. [SyntaxError: functions cannot be labelled](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Function_label)\n    38. [SyntaxError: getter and setter for private name \\#x should either be both static or non-static](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Either_be_both_static_or_non-static)\n    39. [SyntaxError: getter functions must have no arguments](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Getter_no_arguments)\n    40. [SyntaxError: identifier starts immediately after numeric literal](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Identifier_after_number)\n    41. [SyntaxError: illegal character](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Illegal_character)\n    42. [SyntaxError: import declarations may only appear at top level of a module](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/import_decl_module_top_level)\n    43. [SyntaxError: incomplete quantifier in regular expression](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Regex_incomplete_quantifier)\n    44. [SyntaxError: invalid assignment left-hand side](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Invalid_assignment_left-hand_side)\n    45. [SyntaxError: invalid BigInt syntax](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Invalid_BigInt_syntax)\n    46. [SyntaxError: invalid capture group name in regular expression](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Regex_invalid_capture_group_name)\n    47. [SyntaxError: invalid character in class in regular expression](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Regex_invalid_char_in_class)\n    48. [SyntaxError: invalid class set operation in regular expression](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Regex_invalid_class_set_operation)\n    49. [SyntaxError: invalid decimal escape in regular expression](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Regex_invalid_decimal_escape)\n    50. [SyntaxError: invalid identity escape in regular expression](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Regex_invalid_identity_escape)\n    51. [SyntaxError: invalid named capture reference in regular expression](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Regex_invalid_named_capture_reference)\n    52. [SyntaxError: invalid property name in regular expression](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Regex_invalid_property_name)\n    53. [SyntaxError: invalid range in character class](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Regex_invalid_range_in_character_class)\n    54. [SyntaxError: invalid regexp group](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Regex_invalid_group)\n    55. [SyntaxError: invalid regular expression flag \"x\"](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Bad_regexp_flag)\n    56. [SyntaxError: invalid unicode escape in regular expression](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Regex_invalid_unicode_escape)\n    57. [SyntaxError: JSON.parse: bad parsing](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/JSON_bad_parse)\n    58. [SyntaxError: label not found](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Label_not_found)\n    59. [SyntaxError: missing : after property id](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Missing_colon_after_property_id)\n    60. [SyntaxError: missing ) after argument list](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Missing_parenthesis_after_argument_list)\n    61. [SyntaxError: missing ) after condition](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Missing_parenthesis_after_condition)\n    62. [SyntaxError: missing \\] after element list](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Missing_bracket_after_list)\n    63. [SyntaxError: missing } after function body](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Missing_curly_after_function_body)\n    64. [SyntaxError: missing } after property list](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Missing_curly_after_property_list)\n    65. [SyntaxError: missing = in const declaration](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Missing_initializer_in_const)\n    66. [SyntaxError: missing formal parameter](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Missing_formal_parameter)\n    67. [SyntaxError: missing name after . operator](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Missing_name_after_dot_operator)\n    68. [SyntaxError: missing variable name](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/No_variable_name)\n    69. [SyntaxError: negated character class with strings in regular expression](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Regex_negated_char_class_with_strings)\n    70. [SyntaxError: new keyword cannot be used with an optional chain](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Bad_new_optional)\n    71. [SyntaxError: nothing to repeat](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Regex_nothing_to_repeat)\n    72. [SyntaxError: numbers out of order in {} quantifier.](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Regex_numbers_out_of_order_in_quantifier)\n    73. [SyntaxError: octal escape sequences can't be used in untagged template literals or in strict mode code](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Deprecated_octal_escape_sequence)\n    74. [SyntaxError: parameter after rest parameter](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Parameter_after_rest_parameter)\n    75. [SyntaxError: private fields can't be deleted](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Cant_delete_private_fields)\n    76. [SyntaxError: property name \\_\\_proto\\_\\_ appears more than once in object literal](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Duplicate_proto)\n    77. [SyntaxError: raw bracket is not allowed in regular expression with unicode flag](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Regex_raw_bracket)\n    78. [SyntaxError: redeclaration of formal parameter \"x\"](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Redeclared_parameter)\n    79. [SyntaxError: reference to undeclared private field or method \\#x](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Undeclared_private_field_or_method)\n    80. [SyntaxError: rest parameter may not have a default](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Rest_with_default)\n    81. [SyntaxError: return not in function](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Bad_return)\n    82. [SyntaxError: setter functions must have one argument](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Setter_one_argument)\n    83. [SyntaxError: string literal contains an unescaped line break](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/String_literal_EOL)\n    84. [SyntaxError: super() is only valid in derived class constructors](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Bad_super_call)\n    85. [SyntaxError: tagged template cannot be used with optional chain](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Bad_optional_template)\n    86. [SyntaxError: Unexpected '\\#' used outside of class body](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Hash_outside_class)\n    87. [SyntaxError: Unexpected token](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Unexpected_token)\n    88. [SyntaxError: unlabeled break must be inside loop or switch](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Bad_break)\n    89. [SyntaxError: unparenthesized unary expression can't appear on the left-hand side of '\\*\\*'](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Unparenthesized_unary_expr_lhs_exponentiation)\n    90. [SyntaxError: use of super property\/member accesses only valid within methods or eval code within methods](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Bad_super_prop)\n    91. [SyntaxError: Using \/\/@ to indicate sourceURL pragmas is deprecated. Use \/\/\\# instead](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Deprecated_source_map_pragma)\n    92. [TypeError: 'caller', 'callee', and 'arguments' properties may not be accessed](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Deprecated_caller_or_arguments_usage)\n    93. [TypeError: 'x' is not iterable](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/is_not_iterable)\n    94. [TypeError: \"x\" is (not) \"y\"](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Unexpected_type)\n    95. [TypeError: \"x\" is not a constructor](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Not_a_constructor)\n    96. [TypeError: \"x\" is not a function](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Not_a_function)\n    97. [TypeError: \"x\" is not a non-null object](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/No_non-null_object)\n    98. [TypeError: \"x\" is read-only](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Read-only)\n    99. [TypeError: already executing generator](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Already_executing_generator)\n    100. [TypeError: BigInt value can't be serialized in JSON](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/BigInt_not_serializable)\n    101. [TypeError: calling a builtin X constructor without new is forbidden](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Builtin_ctor_no_new)\n    102. [TypeError: can't access\/set private field or method: object is not the right class](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Get_set_missing_private)\n    103. [TypeError: can't assign to property \"x\" on \"y\": not an object](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Cant_assign_to_property)\n    104. [TypeError: can't convert BigInt to number](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Cant_convert_BigInt_to_number)\n    105. [TypeError: can't convert x to BigInt](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Cant_convert_x_to_BigInt)\n    106. [TypeError: can't define property \"x\": \"obj\" is not extensible](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Cant_define_property_object_not_extensible)\n    107. [TypeError: can't delete non-configurable array element](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Non_configurable_array_element)\n    108. [TypeError: can't redefine non-configurable property \"x\"](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Cant_redefine_property)\n    109. [TypeError: can't set prototype of this object](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Cant_set_prototype)\n    110. [TypeError: can't set prototype: it would cause a prototype chain cycle](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Cyclic_prototype)\n    111. [TypeError: cannot use 'in' operator to search for 'x' in 'y'](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/in_operator_no_object)\n    112. [TypeError: class constructors must be invoked with 'new'](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Class_ctor_no_new)\n    113. [TypeError: cyclic object value](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Cyclic_object_value)\n    114. [TypeError: derived class constructor returned invalid value x](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Invalid_derived_return)\n    115. [TypeError: getting private setter-only property](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Private_setter_only)\n    116. [TypeError: Initializing an object twice is an error with private fields\/methods](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Private_double_initialization)\n    117. [TypeError: invalid 'instanceof' operand 'x'](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/invalid_right_hand_side_instanceof_operand)\n    118. [TypeError: invalid Array.prototype.sort argument](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Array_sort_argument)\n    119. [TypeError: invalid assignment to const \"x\"](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Invalid_const_assignment)\n    120. [TypeError: Iterator\/AsyncIterator constructor can't be used directly](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Constructor_cant_be_used_directly)\n    121. [TypeError: matchAll\/replaceAll must be called with a global RegExp](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Requires_global_RegExp)\n    122. [TypeError: More arguments needed](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/More_arguments_needed)\n    123. [TypeError: null\/undefined has no properties](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/No_properties)\n    124. [TypeError: property \"x\" is non-configurable and can't be deleted](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Cant_delete)\n    125. [TypeError: Reduce of empty array with no initial value](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Reduce_of_empty_array_with_no_initial_value)\n    126. [TypeError: setting getter-only property \"x\"](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Getter_only)\n    127. [TypeError: WeakSet key\/WeakMap value 'x' must be an object or an unregistered symbol](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Key_not_weakly_held)\n    128. [TypeError: X.prototype.y called on incompatible type](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Called_on_incompatible_type)\n    129. [URIError: malformed URI sequence](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Malformed_URI)\n    130. [Warning: -file- is being assigned a \/\/\\# sourceMappingURL, but already has one](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Already_has_pragma)\n    131. [Warning: unreachable code after return statement](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Stmt_after_return)\n14. Misc\n    \n    1. [JavaScript technologies overview](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/JavaScript_technologies_overview)\n    2. [Execution model](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Execution_model)\n    3. [Lexical grammar](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Lexical_grammar)\n    4. [Iteration protocols](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Iteration_protocols)\n    5. [Strict mode](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Strict_mode)\n    6. [Template literals](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Template_literals)\n    7. [Trailing commas](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Trailing_commas)\n    8. [Deprecated features](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Deprecated_and_obsolete_features)\n\nYour blueprint for a better internet.\n\nMDN\n\n- [About](https:\/\/developer.mozilla.org\/en-US\/about)\n- [Blog](https:\/\/developer.mozilla.org\/en-US\/blog\/)\n- [Mozilla careers](https:\/\/www.mozilla.org\/en-US\/careers\/listings\/)\n- [Advertise with us](https:\/\/developer.mozilla.org\/en-US\/advertising)\n- [MDN Plus](https:\/\/developer.mozilla.org\/en-US\/plus)\n- [Product help](https:\/\/support.mozilla.org\/products\/mdn-plus)\n\nContribute\n\n- [MDN Community](https:\/\/developer.mozilla.org\/en-US\/community)\n- [Community resources](https:\/\/developer.mozilla.org\/en-US\/docs\/MDN\/Community)\n- [Writing guidelines](https:\/\/developer.mozilla.org\/en-US\/docs\/MDN\/Writing_guidelines)\n- [MDN Discord](https:\/\/developer.mozilla.org\/discord)\n- [MDN on GitHub](https:\/\/github.com\/mdn)\n\nDevelopers\n\n- [Web technologies](https:\/\/developer.mozilla.org\/en-US\/docs\/Web)\n- [Learn web development](https:\/\/developer.mozilla.org\/en-US\/docs\/Learn_web_development)\n- [Guides](https:\/\/developer.mozilla.org\/en-US\/docs\/MDN\/Guides)\n- [Tutorials](https:\/\/developer.mozilla.org\/en-US\/docs\/MDN\/Tutorials)\n- [Glossary](https:\/\/developer.mozilla.org\/en-US\/docs\/Glossary)\n- [Hacks blog](https:\/\/hacks.mozilla.org\/)\n\n- [Website Privacy Notice](https:\/\/www.mozilla.org\/privacy\/websites\/)\n- [Telemetry Settings](https:\/\/www.mozilla.org\/en-US\/privacy\/websites\/data-preferences\/)\n- [Legal](https:\/\/www.mozilla.org\/about\/legal\/terms\/mozilla)\n- [Community Participation Guidelines](https:\/\/www.mozilla.org\/about\/governance\/policies\/participation\/)\n\nVisit [Mozilla Corporation’s](https:\/\/www.mozilla.org\/) not-for-profit parent, the [Mozilla Foundation](https:\/\/foundation.mozilla.org\/).  \n Portions of this content are ©1998–2026 by individual mozilla.org contributors. Content available under [a Creative Commons license](https:\/\/developer.mozilla.org\/docs\/MDN\/Writing_guidelines\/Attrib_copyright_license).","attrs_readable_markdown":"## [Description](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#description)\nFunction values are typically instances of [`Function`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Function). See [`Function`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Function) for information on properties and methods of `Function` objects. Callable values cause [`typeof`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/typeof) to return `\"function\"` instead of `\"object\"`.\n\n**Note:** Not all callable values are `instanceof Function`. For example, the `Function.prototype` object is callable but not an instance of `Function`. You can also manually set the [prototype chain](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Inheritance_and_the_prototype_chain) of your function so it no longer inherits from `Function.prototype`. However, such cases are extremely rare.\n\n### [Return value](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#return_value)\nBy default, if a function's execution doesn't end at a [`return`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/return) statement, or if the `return` keyword doesn't have an expression after it, then the return value is [`undefined`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/undefined). The `return` statement allows you to return an arbitrary value from the function. One function call can only return one value, but you can simulate the effect of returning multiple values by returning an object or array and [destructuring](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Destructuring) the result.\n\n**Note:** Constructors called with [`new`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/new) have a different set of logic to determine their return values.\n\n### [Passing arguments](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#passing_arguments)\n[Parameters and arguments](https:\/\/en.wikipedia.org\/wiki\/Parameter_\$computer_programming\$#Parameters_and_arguments) have slightly different meanings, but in MDN web docs, we often use them interchangeably. For a quick reference:\n\nIn this example, the `num` variable is called the function's *parameter*: it's declared in the parenthesis-enclosed list of the function's definition. The function expects the `num` parameter to be a number — although this is not enforceable in JavaScript without writing runtime validation code. In the `formatNumber(2)` call, the number `2` is the function's *argument*: it's the value that is actually passed to the function in the function call. The argument value can be accessed inside the function body through the corresponding parameter name or the [`arguments`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/arguments) object.\n\nArguments are always [*passed by value*](https:\/\/en.wikipedia.org\/wiki\/Evaluation_strategy#Call_by_reference) and never *passed by reference*. This means that if a function reassigns a parameter, the value won't change outside the function. More precisely, object arguments are [*passed by sharing*](https:\/\/en.wikipedia.org\/wiki\/Evaluation_strategy#Call_by_sharing), which means if the object's properties are mutated, the change will impact the outside of the function. For example:\n\nThe [`this`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/this) keyword refers to the object that the function is accessed on — it does not refer to the currently executing function, so you must refer to the function value by name, even within the function body.\n\n### [Defining functions](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#defining_functions)\nBroadly speaking, JavaScript has four kinds of functions:\n\n- Regular function: can return anything; always runs to completion after invocation\n- Generator function: returns a [`Generator`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Generator) object; can be paused and resumed with the [`yield`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/yield) operator\n- Async function: returns a [`Promise`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Promise); can be paused and resumed with the [`await`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/await) operator\n- Async generator function: returns an [`AsyncGenerator`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/AsyncGenerator) object; both the `await` and `yield` operators can be used\n\nFor every kind of function, there are multiple ways to define it:\n\n[Declaration](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#declaration)\n\n[`function`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/function), [`function*`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/function*), [`async function`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/async_function), [`async function*`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/async_function*)\n\n[Expression](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#expression)\n\n[`function`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/function), [`function*`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/function*), [`async function`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/async_function), [`async function*`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/async_function*)\n\n[Constructor](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#constructor)\n\n[`Function()`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Function\/Function), [`GeneratorFunction()`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/GeneratorFunction\/GeneratorFunction), [`AsyncFunction()`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/AsyncFunction\/AsyncFunction), [`AsyncGeneratorFunction()`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/AsyncGeneratorFunction\/AsyncGeneratorFunction)\n\nIn addition, there are special syntaxes for defining [arrow functions](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/Arrow_functions) and [methods](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/Method_definitions), which provide more precise semantics for their usage. [Classes](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Classes) are conceptually not functions (because they throw an error when called without `new`), but they also inherit from `Function.prototype` and have `typeof MyClass === \"function\"`.\n\nAll syntaxes do approximately the same thing, but there are some subtle behavior differences.\n\n- The `Function()` constructor, `function` expression, and `function` declaration syntaxes create full-fledged function objects, which can be constructed with [`new`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/new). However, arrow functions and methods cannot be constructed. Async functions, generator functions, and async generator functions are not constructible regardless of syntax.\n- The `function` declaration creates functions that are [*hoisted*](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Functions#function_hoisting). Other syntaxes do not hoist the function and the function value is only visible after the definition.\n- The arrow function and `Function()` constructor always create *anonymous* functions, which means they can't easily call themselves recursively. One way to call an arrow function recursively is by assigning it to a variable.\n- The arrow function syntax does not have access to `arguments` or `this`.\n- The `Function()` constructor cannot access any local variables — it only has access to the global scope.\n- The `Function()` constructor causes runtime compilation and is often slower than other syntaxes.\n\nFor `function` expressions, there is a distinction between the function name and the variable the function is assigned to. The function name cannot be changed, while the variable the function is assigned to can be reassigned. The function name can be different from the variable the function is assigned to — they have no relation to each other. The function name can be used only within the function's body. Attempting to use it outside the function's body results in an error (or gets another value, if the same name is declared elsewhere). For example:\n\nOn the other hand, the variable the function is assigned to is limited only by its scope, which is guaranteed to include the scope in which the function is declared.\n\nA function declaration also creates a variable with the same name as the function name. Thus, unlike those defined by function expressions, functions defined by function declarations can be accessed by their name in the scope they were defined in, as well as in their own body.\n\nA function defined by `new Function` will dynamically have its source assembled, which is observable when you serialize it. For example, `console.log(new Function().toString())` gives:\n\nThis is the actual source used to compile the function. However, although the `Function()` constructor will create the function with name `anonymous`, this name is not added to the scope of the body. The body only ever has access to global variables. For example, the following would result in an error:\n\nA function defined by a function expression or by a function declaration inherits the current scope. That is, the function forms a closure. On the other hand, a function defined by a `Function` constructor does not inherit any scope other than the global scope (which all functions inherit).\n\nFunctions defined by function expressions and function declarations are parsed only once, while a function defined by the `Function` constructor parses the string passed to it each and every time the constructor is called. Although a function expression creates a closure every time, the function body is not reparsed, so function expressions are still faster than `new Function(...)`. Therefore the `Function` constructor should generally be avoided whenever possible.\n\nA function declaration may be unintentionally turned into a function expression when it appears in an expression context.\n\nOn the other hand, a function expression may also be turned into a function declaration. An [expression statement](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/Expression_statement) cannot begin with the `function` or `async function` keywords, which is a common mistake when implementing [IIFEs](https:\/\/developer.mozilla.org\/en-US\/docs\/Glossary\/IIFE) (Immediately Invoked Function Expressions).\n\nInstead, start the expression statement with something else, so that the `function` keyword unambiguously starts a function expression. Common options include [grouping](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Grouping) and using [`void`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/void).\n\n### [Function parameters](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#function_parameters)\nEach function parameter is a simple identifier that you can access in the local scope.\n\nThere are three special parameter syntaxes:\n\n- [*Default parameters*](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/Default_parameters) allow formal parameters to be initialized with default values if no value or `undefined` is passed.\n- The [*rest parameter*](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/rest_parameters) allows representing an indefinite number of arguments as an array.\n- [*Destructuring*](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Destructuring) allows unpacking elements from arrays, or properties from objects, into distinct variables.\n\nThere are some consequences if one of the above non-simple parameter syntaxes is used:\n\n- You cannot apply `\"use strict\"` to the function body — this causes a [syntax error](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Errors\/Strict_non_simple_params).\n- Even if the function is not in [strict mode](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Strict_mode), certain strict mode function features apply, including that the [`arguments`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/arguments) object stops syncing with the named parameters, [`arguments.callee`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/arguments\/callee) throws an error when accessed, and duplicate parameter names are not allowed.\n\n### [The arguments object](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#the_arguments_object)\nYou can refer to a function's arguments within the function by using the [`arguments`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/arguments) object.\n\n[`arguments`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/arguments)\n\nAn array-like object containing the arguments passed to the currently executing function.\n\n[`arguments.callee`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/arguments\/callee)\n\nThe currently executing function.\n\n[`arguments.length`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/arguments\/length)\n\nThe number of arguments passed to the function.\n\n### [Getter and setter functions](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#getter_and_setter_functions)\nYou can define accessor properties on any standard built-in object or user-defined object that supports the addition of new properties. Within [object literals](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/Object_initializer) and [classes](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Classes), you can use special syntaxes to define the getter and setter of an accessor property.\n\n[get](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/get)\n\nBinds an object property to a function that will be called when that property is looked up.\n\n[set](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions\/set)\n\nBinds an object property to a function to be called when there is an attempt to set that property.\n\nNote that these syntaxes create an *object property*, not a *method*. The getter and setter functions themselves can only be accessed using reflective APIs such as [`Object.getOwnPropertyDescriptor()`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Object\/getOwnPropertyDescriptor).\n\n### [Block-level functions](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#block-level_functions)\nIn [strict mode](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Strict_mode), functions inside blocks are scoped to that block. Prior to ES2015, block-level functions were forbidden in strict mode.\n\n### [Block-level functions in non-strict code](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#block-level_functions_in_non-strict_code)\nIn a word: **Don't.**\n\nIn non-strict code, function declarations inside blocks behave strangely. For example:\n\nThe semantics of this in strict mode are well-specified — `zero` only ever exists within that scope of the `if` block. If `shouldDefineZero` is false, then `zero` should never be defined, since the block never executes. However, historically, this was left unspecified, so different browsers implemented it differently in non-strict mode. For more information, see the [`function` declaration](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/function#block-level_function_declaration) reference.\n\nA safer way to define functions conditionally is to assign a function expression to a variable:\n## [Examples](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#examples)\n### [Returning a formatted number](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#returning_a_formatted_number)\nThe following function returns a string containing the formatted representation of a number padded with leading zeros.\n\nThe following statements call the `padZeros` function.\n\n### [Determining whether a function exists](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#determining_whether_a_function_exists)\nYou can determine whether a function exists by using the [`typeof`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/typeof) operator. In the following example, a test is performed to determine if the `window` object has a property called `noFunc` that is a function. If so, it is used; otherwise, some other action is taken.\n\nNote that in the `if` test, a reference to `noFunc` is used — there are no parentheses `()` after the function name so the actual function is not called.\n\n## [Specifications](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#specifications)\n| Specification |\n|---|\n| [ECMAScript® 2027 Language Specification \\# sec-function-definitions](https:\/\/tc39.es\/ecma262\/multipage\/ecmascript-language-functions-and-classes.html#sec-function-definitions) |\n## [Browser compatibility](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#browser_compatibility)\n## [See also](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Functions#see_also)\n- [Functions](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Guide\/Functions) guide\n- [Classes](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Classes)\n- [`function`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Statements\/function)\n- [`function` expression](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Operators\/function)\n- [`Function`](https:\/\/developer.mozilla.org\/en-US\/docs\/Web\/JavaScript\/Reference\/Global_Objects\/Function)","meta_canonical":null}

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Page Details

Property	Value
URL	https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions
Last Crawled	2026-04-09 17:19:06 (7 days ago)
First Indexed	2014-09-11 07:01:00 (11 years ago)
HTTP Status Code	200
Meta Title	Functions - JavaScript \| MDN
Meta Description	Generally speaking, a function is a "subprogram" that can be called by code external (or internal, in the case of recursion) to the function. Like the program itself, a function is composed of a sequence of statements called the function body. Values can be passed to a function as parameters, and the function will return a value.
Meta Canonical	null
Boilerpipe Text	Description Function values are typically instances of Function . See Function for information on properties and methods of Function objects. Callable values cause typeof to return "function" instead of "object" . Note: Not all callable values are instanceof Function . For example, the Function.prototype object is callable but not an instance of Function . You can also manually set the prototype chain of your function so it no longer inherits from Function.prototype . However, such cases are extremely rare. Return value By default, if a function's execution doesn't end at a return statement, or if the return keyword doesn't have an expression after it, then the return value is undefined . The return statement allows you to return an arbitrary value from the function. One function call can only return one value, but you can simulate the effect of returning multiple values by returning an object or array and destructuring the result. Note: Constructors called with new have a different set of logic to determine their return values. Passing arguments Parameters and arguments have slightly different meanings, but in MDN web docs, we often use them interchangeably. For a quick reference: In this example, the num variable is called the function's parameter : it's declared in the parenthesis-enclosed list of the function's definition. The function expects the num parameter to be a number — although this is not enforceable in JavaScript without writing runtime validation code. In the formatNumber(2) call, the number 2 is the function's argument : it's the value that is actually passed to the function in the function call. The argument value can be accessed inside the function body through the corresponding parameter name or the arguments object. Arguments are always passed by value and never passed by reference . This means that if a function reassigns a parameter, the value won't change outside the function. More precisely, object arguments are passed by sharing , which means if the object's properties are mutated, the change will impact the outside of the function. For example: The this keyword refers to the object that the function is accessed on — it does not refer to the currently executing function, so you must refer to the function value by name, even within the function body. Defining functions Broadly speaking, JavaScript has four kinds of functions: Regular function: can return anything; always runs to completion after invocation Generator function: returns a Generator object; can be paused and resumed with the yield operator Async function: returns a Promise ; can be paused and resumed with the await operator Async generator function: returns an AsyncGenerator object; both the await and yield operators can be used For every kind of function, there are multiple ways to define it: Declaration function , function* , async function , async function* Expression function , function* , async function , async function* Constructor Function() , GeneratorFunction() , AsyncFunction() , AsyncGeneratorFunction() In addition, there are special syntaxes for defining arrow functions and methods , which provide more precise semantics for their usage. Classes are conceptually not functions (because they throw an error when called without new ), but they also inherit from Function.prototype and have typeof MyClass === "function" . All syntaxes do approximately the same thing, but there are some subtle behavior differences. The Function() constructor, function expression, and function declaration syntaxes create full-fledged function objects, which can be constructed with new . However, arrow functions and methods cannot be constructed. Async functions, generator functions, and async generator functions are not constructible regardless of syntax. The function declaration creates functions that are hoisted . Other syntaxes do not hoist the function and the function value is only visible after the definition. The arrow function and Function() constructor always create anonymous functions, which means they can't easily call themselves recursively. One way to call an arrow function recursively is by assigning it to a variable. The arrow function syntax does not have access to arguments or this . The Function() constructor cannot access any local variables — it only has access to the global scope. The Function() constructor causes runtime compilation and is often slower than other syntaxes. For function expressions, there is a distinction between the function name and the variable the function is assigned to. The function name cannot be changed, while the variable the function is assigned to can be reassigned. The function name can be different from the variable the function is assigned to — they have no relation to each other. The function name can be used only within the function's body. Attempting to use it outside the function's body results in an error (or gets another value, if the same name is declared elsewhere). For example: On the other hand, the variable the function is assigned to is limited only by its scope, which is guaranteed to include the scope in which the function is declared. A function declaration also creates a variable with the same name as the function name. Thus, unlike those defined by function expressions, functions defined by function declarations can be accessed by their name in the scope they were defined in, as well as in their own body. A function defined by new Function will dynamically have its source assembled, which is observable when you serialize it. For example, console.log(new Function().toString()) gives: This is the actual source used to compile the function. However, although the Function() constructor will create the function with name anonymous , this name is not added to the scope of the body. The body only ever has access to global variables. For example, the following would result in an error: A function defined by a function expression or by a function declaration inherits the current scope. That is, the function forms a closure. On the other hand, a function defined by a Function constructor does not inherit any scope other than the global scope (which all functions inherit). Functions defined by function expressions and function declarations are parsed only once, while a function defined by the Function constructor parses the string passed to it each and every time the constructor is called. Although a function expression creates a closure every time, the function body is not reparsed, so function expressions are still faster than new Function(...) . Therefore the Function constructor should generally be avoided whenever possible. A function declaration may be unintentionally turned into a function expression when it appears in an expression context. On the other hand, a function expression may also be turned into a function declaration. An expression statement cannot begin with the function or async function keywords, which is a common mistake when implementing IIFEs (Immediately Invoked Function Expressions). Instead, start the expression statement with something else, so that the function keyword unambiguously starts a function expression. Common options include grouping and using void . Function parameters Each function parameter is a simple identifier that you can access in the local scope. There are three special parameter syntaxes: Default parameters allow formal parameters to be initialized with default values if no value or undefined is passed. The rest parameter allows representing an indefinite number of arguments as an array. Destructuring allows unpacking elements from arrays, or properties from objects, into distinct variables. There are some consequences if one of the above non-simple parameter syntaxes is used: You cannot apply "use strict" to the function body — this causes a syntax error . Even if the function is not in strict mode , certain strict mode function features apply, including that the arguments object stops syncing with the named parameters, arguments.callee throws an error when accessed, and duplicate parameter names are not allowed. The arguments object You can refer to a function's arguments within the function by using the arguments object. arguments An array-like object containing the arguments passed to the currently executing function. arguments.callee The currently executing function. arguments.length The number of arguments passed to the function. Getter and setter functions You can define accessor properties on any standard built-in object or user-defined object that supports the addition of new properties. Within object literals and classes , you can use special syntaxes to define the getter and setter of an accessor property. get Binds an object property to a function that will be called when that property is looked up. set Binds an object property to a function to be called when there is an attempt to set that property. Note that these syntaxes create an object property , not a method . The getter and setter functions themselves can only be accessed using reflective APIs such as Object.getOwnPropertyDescriptor() . Block-level functions In strict mode , functions inside blocks are scoped to that block. Prior to ES2015, block-level functions were forbidden in strict mode. Block-level functions in non-strict code In a word: Don't. In non-strict code, function declarations inside blocks behave strangely. For example: The semantics of this in strict mode are well-specified — zero only ever exists within that scope of the if block. If shouldDefineZero is false, then zero should never be defined, since the block never executes. However, historically, this was left unspecified, so different browsers implemented it differently in non-strict mode. For more information, see the function declaration reference. A safer way to define functions conditionally is to assign a function expression to a variable: Examples Returning a formatted number The following function returns a string containing the formatted representation of a number padded with leading zeros. The following statements call the padZeros function. Determining whether a function exists You can determine whether a function exists by using the typeof operator. In the following example, a test is performed to determine if the window object has a property called noFunc that is a function. If so, it is used; otherwise, some other action is taken. Note that in the if test, a reference to noFunc is used — there are no parentheses () after the function name so the actual function is not called. Specifications Specification ECMAScript® 2027 Language Specification # sec-function-definitions Browser compatibility See also Functions guide Classes function function expression Function
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[Web](https://developer.mozilla.org/en-US/docs/Web) 2. [JavaScript](https://developer.mozilla.org/en-US/docs/Web/JavaScript) 3. [Reference](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference) 4. [Functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions) # Functions Baseline Widely available \* This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015. \* Some parts of this feature may have varying levels of support. - [Learn more](https://developer.mozilla.org/en-US/docs/Glossary/Baseline/Compatibility) - [See full compatibility](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#browser_compatibility) - [Report feedback](https://survey.alchemer.com/s3/7634825/MDN-baseline-feedback?page=%2Fen-US%2Fdocs%2FWeb%2FJavaScript%2FReference%2FFunctions&level=high) Generally speaking, a function is a "subprogram" that can be called by code external (or internal, in the case of recursion) to the function. Like the program itself, a function is composed of a sequence of statements called the function body. Values can be passed to a function as parameters, and the function will return a value. In JavaScript, functions are [first-class objects](https://developer.mozilla.org/en-US/docs/Glossary/First-class_Function), because they can be passed to other functions, returned from functions, and assigned to variables and properties. They can also have properties and methods just like any other object. What distinguishes them from other objects is that functions can be called. For more examples and explanations, see the [JavaScript guide about functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions). ## In this article - [Description](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#description) - [Examples](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#examples) - [Specifications](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#specifications) - [Browser compatibility](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#browser_compatibility) - [See also](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#see_also) ## [Description](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#description) Function values are typically instances of [`Function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function). See [`Function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function) for information on properties and methods of `Function` objects. Callable values cause [`typeof`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof) to return `"function"` instead of `"object"`. Note: Not all callable values are `instanceof Function`. For example, the `Function.prototype` object is callable but not an instance of `Function`. You can also manually set the [prototype chain](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Inheritance_and_the_prototype_chain) of your function so it no longer inherits from `Function.prototype`. However, such cases are extremely rare. ### [Return value](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#return_value) By default, if a function's execution doesn't end at a [`return`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/return) statement, or if the `return` keyword doesn't have an expression after it, then the return value is [`undefined`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined). The `return` statement allows you to return an arbitrary value from the function. One function call can only return one value, but you can simulate the effect of returning multiple values by returning an object or array and [destructuring](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring) the result. Note: Constructors called with [`new`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/new) have a different set of logic to determine their return values. ### [Passing arguments](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#passing_arguments) [Parameters and arguments](https://en.wikipedia.org/wiki/Parameter_$computer_programming$#Parameters_and_arguments) have slightly different meanings, but in MDN web docs, we often use them interchangeably. For a quick reference: In this example, the `num` variable is called the function's parameter: it's declared in the parenthesis-enclosed list of the function's definition. The function expects the `num` parameter to be a number — although this is not enforceable in JavaScript without writing runtime validation code. In the `formatNumber(2)` call, the number `2` is the function's argument: it's the value that is actually passed to the function in the function call. The argument value can be accessed inside the function body through the corresponding parameter name or the [`arguments`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments) object. Arguments are always [passed by value](https://en.wikipedia.org/wiki/Evaluation_strategy#Call_by_reference) and never passed by reference. This means that if a function reassigns a parameter, the value won't change outside the function. More precisely, object arguments are [passed by sharing](https://en.wikipedia.org/wiki/Evaluation_strategy#Call_by_sharing), which means if the object's properties are mutated, the change will impact the outside of the function. For example: The [`this`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this) keyword refers to the object that the function is accessed on — it does not refer to the currently executing function, so you must refer to the function value by name, even within the function body. ### [Defining functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#defining_functions) Broadly speaking, JavaScript has four kinds of functions: - Regular function: can return anything; always runs to completion after invocation - Generator function: returns a [`Generator`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Generator) object; can be paused and resumed with the [`yield`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/yield) operator - Async function: returns a [`Promise`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise); can be paused and resumed with the [`await`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await) operator - Async generator function: returns an [`AsyncGenerator`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncGenerator) object; both the `await` and `yield` operators can be used For every kind of function, there are multiple ways to define it: [Declaration](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#declaration) [`function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function), [`function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function), [`async function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function), [`async function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function) [Expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#expression) [`function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/function), [`function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/function), [`async function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/async_function), [`async function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/async_function) [Constructor](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#constructor) [`Function()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/Function), [`GeneratorFunction()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/GeneratorFunction/GeneratorFunction), [`AsyncFunction()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncFunction/AsyncFunction), [`AsyncGeneratorFunction()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncGeneratorFunction/AsyncGeneratorFunction) In addition, there are special syntaxes for defining [arrow functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions) and [methods](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Method_definitions), which provide more precise semantics for their usage. [Classes](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes) are conceptually not functions (because they throw an error when called without `new`), but they also inherit from `Function.prototype` and have `typeof MyClass === "function"`. All syntaxes do approximately the same thing, but there are some subtle behavior differences. - The `Function()` constructor, `function` expression, and `function` declaration syntaxes create full-fledged function objects, which can be constructed with [`new`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/new). However, arrow functions and methods cannot be constructed. Async functions, generator functions, and async generator functions are not constructible regardless of syntax. - The `function` declaration creates functions that are [hoisted](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions#function_hoisting). Other syntaxes do not hoist the function and the function value is only visible after the definition. - The arrow function and `Function()` constructor always create anonymous functions, which means they can't easily call themselves recursively. One way to call an arrow function recursively is by assigning it to a variable. - The arrow function syntax does not have access to `arguments` or `this`. - The `Function()` constructor cannot access any local variables — it only has access to the global scope. - The `Function()` constructor causes runtime compilation and is often slower than other syntaxes. For `function` expressions, there is a distinction between the function name and the variable the function is assigned to. The function name cannot be changed, while the variable the function is assigned to can be reassigned. The function name can be different from the variable the function is assigned to — they have no relation to each other. The function name can be used only within the function's body. Attempting to use it outside the function's body results in an error (or gets another value, if the same name is declared elsewhere). For example: On the other hand, the variable the function is assigned to is limited only by its scope, which is guaranteed to include the scope in which the function is declared. A function declaration also creates a variable with the same name as the function name. Thus, unlike those defined by function expressions, functions defined by function declarations can be accessed by their name in the scope they were defined in, as well as in their own body. A function defined by `new Function` will dynamically have its source assembled, which is observable when you serialize it. For example, `console.log(new Function().toString())` gives: This is the actual source used to compile the function. However, although the `Function()` constructor will create the function with name `anonymous`, this name is not added to the scope of the body. The body only ever has access to global variables. For example, the following would result in an error: A function defined by a function expression or by a function declaration inherits the current scope. That is, the function forms a closure. On the other hand, a function defined by a `Function` constructor does not inherit any scope other than the global scope (which all functions inherit). Functions defined by function expressions and function declarations are parsed only once, while a function defined by the `Function` constructor parses the string passed to it each and every time the constructor is called. Although a function expression creates a closure every time, the function body is not reparsed, so function expressions are still faster than `new Function(...)`. Therefore the `Function` constructor should generally be avoided whenever possible. A function declaration may be unintentionally turned into a function expression when it appears in an expression context. On the other hand, a function expression may also be turned into a function declaration. An [expression statement](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/Expression_statement) cannot begin with the `function` or `async function` keywords, which is a common mistake when implementing [IIFEs](https://developer.mozilla.org/en-US/docs/Glossary/IIFE) (Immediately Invoked Function Expressions). Instead, start the expression statement with something else, so that the `function` keyword unambiguously starts a function expression. Common options include [grouping](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Grouping) and using [`void`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/void). ### [Function parameters](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#function_parameters) Each function parameter is a simple identifier that you can access in the local scope. There are three special parameter syntaxes: - [Default parameters](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Default_parameters) allow formal parameters to be initialized with default values if no value or `undefined` is passed. - The [rest parameter](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/rest_parameters) allows representing an indefinite number of arguments as an array. - [Destructuring](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring) allows unpacking elements from arrays, or properties from objects, into distinct variables. There are some consequences if one of the above non-simple parameter syntaxes is used: - You cannot apply `"use strict"` to the function body — this causes a [syntax error](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Strict_non_simple_params). - Even if the function is not in [strict mode](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode), certain strict mode function features apply, including that the [`arguments`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments) object stops syncing with the named parameters, [`arguments.callee`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments/callee) throws an error when accessed, and duplicate parameter names are not allowed. ### [The arguments object](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#the_arguments_object) You can refer to a function's arguments within the function by using the [`arguments`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments) object. [`arguments`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments) An array-like object containing the arguments passed to the currently executing function. [`arguments.callee`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments/callee) The currently executing function. [`arguments.length`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments/length) The number of arguments passed to the function. ### [Getter and setter functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#getter_and_setter_functions) You can define accessor properties on any standard built-in object or user-defined object that supports the addition of new properties. Within [object literals](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Object_initializer) and [classes](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes), you can use special syntaxes to define the getter and setter of an accessor property. [get](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/get) Binds an object property to a function that will be called when that property is looked up. [set](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/set) Binds an object property to a function to be called when there is an attempt to set that property. Note that these syntaxes create an object property, not a method. The getter and setter functions themselves can only be accessed using reflective APIs such as [`Object.getOwnPropertyDescriptor()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/getOwnPropertyDescriptor). ### [Block-level functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#block-level_functions) In [strict mode](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode), functions inside blocks are scoped to that block. Prior to ES2015, block-level functions were forbidden in strict mode. ### [Block-level functions in non-strict code](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#block-level_functions_in_non-strict_code) In a word: Don't. In non-strict code, function declarations inside blocks behave strangely. For example: The semantics of this in strict mode are well-specified — `zero` only ever exists within that scope of the `if` block. If `shouldDefineZero` is false, then `zero` should never be defined, since the block never executes. However, historically, this was left unspecified, so different browsers implemented it differently in non-strict mode. For more information, see the [`function` declaration](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function#block-level_function_declaration) reference. A safer way to define functions conditionally is to assign a function expression to a variable: ## [Examples](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#examples) ### [Returning a formatted number](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#returning_a_formatted_number) The following function returns a string containing the formatted representation of a number padded with leading zeros. The following statements call the `padZeros` function. ### [Determining whether a function exists](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#determining_whether_a_function_exists) You can determine whether a function exists by using the [`typeof`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof) operator. In the following example, a test is performed to determine if the `window` object has a property called `noFunc` that is a function. If so, it is used; otherwise, some other action is taken. Note that in the `if` test, a reference to `noFunc` is used — there are no parentheses `()` after the function name so the actual function is not called. ## [Specifications](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#specifications) \| Specification \| \|---\| \| [ECMAScript® 2027 Language Specification \# sec-function-definitions](https://tc39.es/ecma262/multipage/ecmascript-language-functions-and-classes.html#sec-function-definitions) \| ## [Browser compatibility](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#browser_compatibility) ## [See also](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#see_also) - [Functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions) guide - [Classes](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes) - [`function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function) - [`function` expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/function) - [`Function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function) ## Help improve MDN [Learn how to contribute](https://developer.mozilla.org/en-US/docs/MDN/Community/Getting_started) This page was last modified on Jul 8, 2025 by [MDN contributors](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/contributors.txt). [View this page on GitHub](https://github.com/mdn/content/blob/main/files/en-us/web/javascript/reference/functions/index.md?plain=1 "Folder: en-us/web/javascript/reference/functions (Opens in a new tab)") • [Report a problem with this content](https://github.com/mdn/content/issues/new?template=page-report.yml&mdn-url=https%3A%2F%2Fdeveloper.mozilla.org%2Fen-US%2Fdocs%2FWeb%2FJavaScript%2FReference%2FFunctions&metadata=%3C%21--+Do+not+make+changes+below+this+line+--%3E%0A%3Cdetails%3E%0A%3Csummary%3EPage+report+details%3C%2Fsummary%3E%0A%0A+Folder%3A+%60en-us%2Fweb%2Fjavascript%2Freference%2Ffunctions%60%0A+MDN+URL%3A+https%3A%2F%2Fdeveloper.mozilla.org%2Fen-US%2Fdocs%2FWeb%2FJavaScript%2FReference%2FFunctions%0A+GitHub+URL%3A+https%3A%2F%2Fgithub.com%2Fmdn%2Fcontent%2Fblob%2Fmain%2Ffiles%2Fen-us%2Fweb%2Fjavascript%2Freference%2Ffunctions%2Findex.md%0A+Last+commit%3A+https%3A%2F%2Fgithub.com%2Fmdn%2Fcontent%2Fcommit%2Ffad67be4431d8e6c2a89ac880735233aa76c41d4%0A+Document+last+modified%3A+2025-07-08T13%3A18%3A45.000Z%0A%0A%3C%2Fdetails%3E "This will take you to GitHub to file a new issue.") 1. [JavaScript](https://developer.mozilla.org/en-US/docs/Web/JavaScript) 2. Tutorials and guides 3. [JavaScript Guide](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide) 1. [Introduction](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Introduction) 2. [Grammar and types](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Grammar_and_types) 3. [Control flow and error handling](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Control_flow_and_error_handling) 4. [Loops and iteration](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Loops_and_iteration) 5. [Functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions) 6. [Expressions and operators](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Expressions_and_operators) 7. [Numbers and strings](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Numbers_and_strings) 8. [Representing dates & times](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Representing_dates_times) 9. [Regular expressions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_expressions) 10. [Indexed collections](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Indexed_collections) 11. [Keyed collections](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Keyed_collections) 12. [Working with objects](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Working_with_objects) 13. [Using classes](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Using_classes) 14. [Using promises](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Using_promises) 15. [JavaScript typed arrays](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Typed_arrays) 16. [Iterators and generators](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Iterators_and_generators) 17. [Resource management](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Resource_management) 18. [Internationalization](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Internationalization) 19. [JavaScript modules](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules) 4. Intermediate 1. [Language overview](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Language_overview) 2. [JavaScript data structures](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Data_structures) 3. [Equality comparisons and sameness](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Equality_comparisons_and_sameness) 4. [Enumerability and ownership of properties](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Enumerability_and_ownership_of_properties) 5. [Closures](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Closures) 5. Advanced 1. [Inheritance and the prototype chain](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Inheritance_and_the_prototype_chain) 2. [Meta programming](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Meta_programming) 3. [Memory Management](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Memory_management) 6. [References](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference) 7. [Built-in objects](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects) 1. [AggregateError](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/AggregateError) 2. [Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array) 3. [ArrayBuffer](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer) 4. [AsyncDisposableStack](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncDisposableStack) 5. [AsyncFunction](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncFunction) 6. [AsyncGenerator](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncGenerator) 7. [AsyncGeneratorFunction](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncGeneratorFunction) 8. [AsyncIterator](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncIterator) 9. [Atomics](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Atomics) 10. [BigInt](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt) 11. [BigInt64Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt64Array) 12. [BigUint64Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigUint64Array) 13. [Boolean](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Boolean) 14. [DataView](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DataView) 15. [Date](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date) 16. [decodeURI()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/decodeURI) 17. [decodeURIComponent()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/decodeURIComponent) 18. [DisposableStack](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DisposableStack) 19. [encodeURI()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/encodeURI) 20. [encodeURIComponent()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/encodeURIComponent) 21. [Error](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error) 22. [escape()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/escape) Deprecated 23. [eval()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval) 24. [EvalError](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/EvalError) 25. [FinalizationRegistry](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/FinalizationRegistry) 26. [Float16Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Float16Array) 27. [Float32Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Float32Array) 28. [Float64Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Float64Array) 29. [Function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function) 30. [Generator](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Generator) 31. [GeneratorFunction](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/GeneratorFunction) 32. [globalThis](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/globalThis) 33. [Infinity](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Infinity) 34. [Int8Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Int8Array) 35. [Int16Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Int16Array) 36. [Int32Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Int32Array) 37. [InternalError](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/InternalError) Non-standard 38. [Intl](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl) 39. [isFinite()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/isFinite) 40. [isNaN()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/isNaN) 41. [Iterator](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Iterator) 42. [JSON](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON) 43. [Map](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map) 44. [Math](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math) 45. [NaN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN) 46. [Number](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number) 47. [Object](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object) 48. [parseFloat()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/parseFloat) 49. [parseInt()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/parseInt) 50. [Promise](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise) 51. [Proxy](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy) 52. [RangeError](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RangeError) 53. [ReferenceError](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ReferenceError) 54. [Reflect](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Reflect) 55. [RegExp](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp) 56. [Set](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set) 57. [SharedArrayBuffer](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer) 58. [String](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String) 59. [SuppressedError](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SuppressedError) 60. [Symbol](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol) 61. [SyntaxError](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SyntaxError) 62. [Temporal](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Temporal) 63. [TypedArray](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray) 64. [TypeError](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypeError) 65. [Uint8Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array) 66. [Uint8ClampedArray](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8ClampedArray) 67. [Uint16Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint16Array) 68. [Uint32Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint32Array) 69. [undefined](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined) 70. [unescape()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/unescape) Deprecated 71. [URIError](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/URIError) 72. [WeakMap](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakMap) 73. [WeakRef](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakRef) 74. [WeakSet](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakSet) 8. [Expressions & operators](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators) 1. [Addition (+)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Addition) 2. [Addition assignment (+=)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Addition_assignment) 3. [Assignment (=)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Assignment) 4. [async function expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/async_function) 5. [async function\ expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/async_function) 6. [await](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await) 7. [Bitwise AND (&)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_AND) 8. [Bitwise AND assignment (&=)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_AND_assignment) 9. [Bitwise NOT (~)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_NOT) 10. [Bitwise OR (\\|)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_OR) 11. [Bitwise OR assignment (\\|=)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_OR_assignment) 12. [Bitwise XOR (^)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_XOR) 13. [Bitwise XOR assignment (^=)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_XOR_assignment) 14. [class expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/class) 15. [Comma operator (,)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comma_operator) 16. [Conditional (ternary) operator](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Conditional_operator) 17. [Decrement (--)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Decrement) 18. [delete](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/delete) 19. [Destructuring](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring) 20. [Division (/)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Division) 21. [Division assignment (/=)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Division_assignment) 22. [Equality (==)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Equality) 23. [Exponentiation (\\)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Exponentiation) 24. [Exponentiation assignment (\\=)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Exponentiation_assignment) 25. [function expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/function) 26. [function\ expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/function) 27. [Greater than (\>)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Greater_than) 28. [Greater than or equal (\>=)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Greater_than_or_equal) 29. [Grouping operator ( )](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Grouping) 30. [import.meta](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/import.meta) 1. [import.meta.resolve()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/import.meta/resolve) 31. [import()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/import) 32. [in](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/in) 33. [Increment (++)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Increment) 34. [Inequality (!=)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Inequality) 35. [instanceof](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/instanceof) 36. [Left shift (\<\<)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Left_shift) 37. [Left shift assignment (\<\<=)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Left_shift_assignment) 38. [Less than (\<)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Less_than) 39. [Less than or equal (\<=)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Less_than_or_equal) 40. [Logical AND (&&)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_AND) 41. [Logical AND assignment (&&=)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_AND_assignment) 42. [Logical NOT (!)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_NOT) 43. [Logical OR (\\|\\|)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_OR) 44. [Logical OR assignment (\\|\\|=)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_OR_assignment) 45. [Multiplication (\)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Multiplication) 46. [Multiplication assignment (\=)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Multiplication_assignment) 47. [new](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/new) 48. [new.target](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/new.target) 49. [null](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/null) 50. [Nullish coalescing assignment (??=)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_assignment) 51. [Nullish coalescing operator (??)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing) 52. [Object initializer](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Object_initializer) 53. [Operator precedence](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Operator_precedence) 54. [Optional chaining (?.)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining) 55. [Property accessors](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Property_accessors) 56. [Remainder (%)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Remainder) 57. [Remainder assignment (%=)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Remainder_assignment) 58. [Right shift (\>\>)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Right_shift) 59. [Right shift assignment (\>\>=)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Right_shift_assignment) 60. [Spread syntax (...)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax) 61. [Strict equality (===)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Strict_equality) 62. [Strict inequality (!==)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Strict_inequality) 63. [Subtraction (-)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Subtraction) 64. [Subtraction assignment (-=)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Subtraction_assignment) 65. [super](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/super) 66. [this](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this) 67. [typeof](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof) 68. [Unary negation (-)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Unary_negation) 69. [Unary plus (+)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Unary_plus) 70. [Unsigned right shift (\>\>\>)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Unsigned_right_shift) 71. [Unsigned right shift assignment (\>\>\>=)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Unsigned_right_shift_assignment) 72. [void operator](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/void) 73. [yield](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/yield) 74. [yield\](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/yield) 9. [Statements & declarations](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements) 1. [async function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function) 2. [async function\](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function) 3. [await using](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/await_using) 4. [Block statement](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/block) 5. [break](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/break) 6. [class](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/class) 7. [const](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/const) 8. [continue](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/continue) 9. [debugger](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/debugger) 10. [do...while](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/do...while) 11. [Empty statement](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/Empty) 12. [export](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/export) 13. [Expression statement](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/Expression_statement) 14. [for](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for) 15. [for await...of](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for-await...of) 16. [for...in](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...in) 17. [for...of](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...of) 18. [function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function) 19. [function\](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function) 20. [if...else](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/if...else) 21. [import](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import) 1. [Import attributes](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import/with) 22. [Labeled statement](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/label) 23. [let](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let) 24. [return](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/return) 25. [switch](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/switch) 26. [throw](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/throw) 27. [try...catch](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/try...catch) 28. [using](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/using) 29. [var](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var) 30. [while](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/while) 31. [with](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/with) Deprecated 10. [Functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions)* 1. [Arrow function expressions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions) 2. [Default parameters](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Default_parameters) 3. [get](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/get) 4. [Method definitions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Method_definitions) 5. [Rest parameters](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/rest_parameters) 6. [set](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/set) 7. [The arguments object](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments) 1. [\[Symbol.iterator\]()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments/Symbol.iterator) 2. [callee](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments/callee) Deprecated 3. [length](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments/length) 11. [Classes](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes) 1. [constructor](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/constructor) 2. [extends](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/extends) 3. [Private elements](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/Private_elements) 4. [Public class fields](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/Public_class_fields) 5. [static](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/static) 6. [Static initialization blocks](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/Static_initialization_blocks) 12. [Regular expressions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Regular_expressions) 1. [Backreference: \\1, \\2](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Backreference) 2. [Capturing group: (...)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Capturing_group) 3. [Character class escape: \\d, \\D, \\w, \\W, \\s, \\S](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Character_class_escape) 4. [Character class: \[...\], \[^...\]](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Character_class) 5. [Character escape: \\n, \\u{...}](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Character_escape) 6. [Disjunction: \\|](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Disjunction) 7. [Input boundary assertion: ^, \$](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Input_boundary_assertion) 8. [Literal character: a, b](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Literal_character) 9. [Lookahead assertion: (?=...), (?!...)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Lookahead_assertion) 10. [Lookbehind assertion: (?\<=...), (?\<!...)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Lookbehind_assertion) 11. [Modifier: (?ims-ims:...)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Modifier) 12. [Named backreference: \\k\<name\>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Named_backreference) 13. [Named capturing group: (?\<name\>...)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Named_capturing_group) 14. [Non-capturing group: (?:...)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Non-capturing_group) 15. [Quantifier: \, +, ?, {n}, {n,}, {n,m}](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Quantifier) 16. [Unicode character class escape: \\p{...}, \\P{...}](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Unicode_character_class_escape) 17. [Wildcard: .](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Wildcard) 18. [Word boundary assertion: \\b, \\B](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Word_boundary_assertion) 13. [Errors](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors) 1. [AggregateError: No Promise in Promise.any was resolved](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Promise_any_all_rejected) 2. [Error: Permission denied to access property "x"](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Property_access_denied) 3. [InternalError: too much recursion](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Too_much_recursion) 4. [RangeError: argument is not a valid code point](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Not_a_valid_code_point) 5. [RangeError: BigInt division by zero](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/BigInt_division_by_zero) 6. [RangeError: BigInt negative exponent](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/BigInt_negative_exponent) 7. [RangeError: form must be one of 'NFC', 'NFD', 'NFKC', or 'NFKD'](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Form_must_be_one_of) 8. [RangeError: invalid array length](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_array_length) 9. [RangeError: invalid date](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_date) 10. [RangeError: precision is out of range](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Precision_range) 11. [RangeError: radix must be an integer](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Bad_radix) 12. [RangeError: repeat count must be less than infinity](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Resulting_string_too_large) 13. [RangeError: repeat count must be non-negative](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Negative_repetition_count) 14. [RangeError: x can't be converted to BigInt because it isn't an integer](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Cant_be_converted_to_BigInt_because_it_isnt_an_integer) 15. [ReferenceError: "x" is not defined](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Not_defined) 16. [ReferenceError: assignment to undeclared variable "x"](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Undeclared_var) 17. [ReferenceError: can't access lexical declaration 'X' before initialization](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Cant_access_lexical_declaration_before_init) 18. [ReferenceError: must call super constructor before using 'this' in derived class constructor](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Super_not_called) 19. [ReferenceError: super() called twice in derived class constructor](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Super_called_twice) 20. [SyntaxError: 'arguments'/'eval' can't be defined or assigned to in strict mode code](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Bad_strict_arguments_eval) 21. [SyntaxError: "0"-prefixed octal literals are deprecated](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Deprecated_octal_literal) 22. [SyntaxError: "use strict" not allowed in function with non-simple parameters](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Strict_non_simple_params) 23. [SyntaxError: "x" is a reserved identifier](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Reserved_identifier) 24. [SyntaxError: \\ at end of pattern](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Regex_backslash_at_end_of_pattern) 25. [SyntaxError: a declaration in the head of a for-of loop can't have an initializer](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_for-of_initializer) 26. [SyntaxError: applying the 'delete' operator to an unqualified name is deprecated](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Delete_in_strict_mode) 27. [SyntaxError: arguments is not valid in fields](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Arguments_not_allowed) 28. [SyntaxError: await is only valid in async functions, async generators and modules](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Bad_await) 29. [SyntaxError: await/yield expression can't be used in parameter](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/await_yield_in_parameter) 30. [SyntaxError: cannot use ?? unparenthesized within \\|\\| and && expressions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Cant_use_nullish_coalescing_unparenthesized) 31. [SyntaxError: character class escape cannot be used in class range in regular expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Regex_character_class_escape_in_class_range) 32. [SyntaxError: continue must be inside loop](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Bad_continue) 33. [SyntaxError: duplicate capture group name in regular expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Regex_duplicate_capture_group_name) 34. [SyntaxError: duplicate formal argument x](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Duplicate_parameter) 35. [SyntaxError: for-in loop head declarations may not have initializers](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_for-in_initializer) 36. [SyntaxError: function statement requires a name](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Unnamed_function_statement) 37. [SyntaxError: functions cannot be labelled](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Function_label) 38. [SyntaxError: getter and setter for private name \#x should either be both static or non-static](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Either_be_both_static_or_non-static) 39. [SyntaxError: getter functions must have no arguments](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Getter_no_arguments) 40. [SyntaxError: identifier starts immediately after numeric literal](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Identifier_after_number) 41. [SyntaxError: illegal character](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Illegal_character) 42. [SyntaxError: import declarations may only appear at top level of a module](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/import_decl_module_top_level) 43. [SyntaxError: incomplete quantifier in regular expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Regex_incomplete_quantifier) 44. [SyntaxError: invalid assignment left-hand side](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_assignment_left-hand_side) 45. [SyntaxError: invalid BigInt syntax](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_BigInt_syntax) 46. [SyntaxError: invalid capture group name in regular expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_capture_group_name) 47. [SyntaxError: invalid character in class in regular expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_char_in_class) 48. [SyntaxError: invalid class set operation in regular expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_class_set_operation) 49. [SyntaxError: invalid decimal escape in regular expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_decimal_escape) 50. [SyntaxError: invalid identity escape in regular expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_identity_escape) 51. [SyntaxError: invalid named capture reference in regular expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_named_capture_reference) 52. [SyntaxError: invalid property name in regular expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_property_name) 53. [SyntaxError: invalid range in character class](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_range_in_character_class) 54. [SyntaxError: invalid regexp group](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_group) 55. [SyntaxError: invalid regular expression flag "x"](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Bad_regexp_flag) 56. [SyntaxError: invalid unicode escape in regular expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Regex_invalid_unicode_escape) 57. [SyntaxError: JSON.parse: bad parsing](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/JSON_bad_parse) 58. [SyntaxError: label not found](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Label_not_found) 59. [SyntaxError: missing : after property id](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Missing_colon_after_property_id) 60. [SyntaxError: missing ) after argument list](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Missing_parenthesis_after_argument_list) 61. [SyntaxError: missing ) after condition](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Missing_parenthesis_after_condition) 62. [SyntaxError: missing \] after element list](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Missing_bracket_after_list) 63. [SyntaxError: missing } after function body](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Missing_curly_after_function_body) 64. [SyntaxError: missing } after property list](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Missing_curly_after_property_list) 65. [SyntaxError: missing = in const declaration](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Missing_initializer_in_const) 66. [SyntaxError: missing formal parameter](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Missing_formal_parameter) 67. [SyntaxError: missing name after . operator](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Missing_name_after_dot_operator) 68. [SyntaxError: missing variable name](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/No_variable_name) 69. [SyntaxError: negated character class with strings in regular expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Regex_negated_char_class_with_strings) 70. [SyntaxError: new keyword cannot be used with an optional chain](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Bad_new_optional) 71. [SyntaxError: nothing to repeat](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Regex_nothing_to_repeat) 72. [SyntaxError: numbers out of order in {} quantifier.](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Regex_numbers_out_of_order_in_quantifier) 73. [SyntaxError: octal escape sequences can't be used in untagged template literals or in strict mode code](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Deprecated_octal_escape_sequence) 74. [SyntaxError: parameter after rest parameter](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Parameter_after_rest_parameter) 75. [SyntaxError: private fields can't be deleted](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Cant_delete_private_fields) 76. [SyntaxError: property name \_\_proto\_\_ appears more than once in object literal](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Duplicate_proto) 77. [SyntaxError: raw bracket is not allowed in regular expression with unicode flag](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Regex_raw_bracket) 78. [SyntaxError: redeclaration of formal parameter "x"](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Redeclared_parameter) 79. [SyntaxError: reference to undeclared private field or method \#x](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Undeclared_private_field_or_method) 80. [SyntaxError: rest parameter may not have a default](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Rest_with_default) 81. [SyntaxError: return not in function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Bad_return) 82. [SyntaxError: setter functions must have one argument](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Setter_one_argument) 83. [SyntaxError: string literal contains an unescaped line break](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/String_literal_EOL) 84. [SyntaxError: super() is only valid in derived class constructors](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Bad_super_call) 85. [SyntaxError: tagged template cannot be used with optional chain](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Bad_optional_template) 86. [SyntaxError: Unexpected '\#' used outside of class body](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Hash_outside_class) 87. [SyntaxError: Unexpected token](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Unexpected_token) 88. [SyntaxError: unlabeled break must be inside loop or switch](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Bad_break) 89. [SyntaxError: unparenthesized unary expression can't appear on the left-hand side of '\\*'](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Unparenthesized_unary_expr_lhs_exponentiation) 90. [SyntaxError: use of super property/member accesses only valid within methods or eval code within methods](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Bad_super_prop) 91. [SyntaxError: Using //@ to indicate sourceURL pragmas is deprecated. Use //\# instead](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Deprecated_source_map_pragma) 92. [TypeError: 'caller', 'callee', and 'arguments' properties may not be accessed](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Deprecated_caller_or_arguments_usage) 93. [TypeError: 'x' is not iterable](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/is_not_iterable) 94. [TypeError: "x" is (not) "y"](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Unexpected_type) 95. [TypeError: "x" is not a constructor](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Not_a_constructor) 96. [TypeError: "x" is not a function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Not_a_function) 97. [TypeError: "x" is not a non-null object](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/No_non-null_object) 98. [TypeError: "x" is read-only](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Read-only) 99. [TypeError: already executing generator](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Already_executing_generator) 100. [TypeError: BigInt value can't be serialized in JSON](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/BigInt_not_serializable) 101. [TypeError: calling a builtin X constructor without new is forbidden](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Builtin_ctor_no_new) 102. [TypeError: can't access/set private field or method: object is not the right class](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Get_set_missing_private) 103. [TypeError: can't assign to property "x" on "y": not an object](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Cant_assign_to_property) 104. [TypeError: can't convert BigInt to number](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Cant_convert_BigInt_to_number) 105. [TypeError: can't convert x to BigInt](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Cant_convert_x_to_BigInt) 106. [TypeError: can't define property "x": "obj" is not extensible](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Cant_define_property_object_not_extensible) 107. [TypeError: can't delete non-configurable array element](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Non_configurable_array_element) 108. [TypeError: can't redefine non-configurable property "x"](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Cant_redefine_property) 109. [TypeError: can't set prototype of this object](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Cant_set_prototype) 110. [TypeError: can't set prototype: it would cause a prototype chain cycle](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Cyclic_prototype) 111. [TypeError: cannot use 'in' operator to search for 'x' in 'y'](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/in_operator_no_object) 112. [TypeError: class constructors must be invoked with 'new'](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Class_ctor_no_new) 113. [TypeError: cyclic object value](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Cyclic_object_value) 114. [TypeError: derived class constructor returned invalid value x](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_derived_return) 115. [TypeError: getting private setter-only property](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Private_setter_only) 116. [TypeError: Initializing an object twice is an error with private fields/methods](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Private_double_initialization) 117. [TypeError: invalid 'instanceof' operand 'x'](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/invalid_right_hand_side_instanceof_operand) 118. [TypeError: invalid Array.prototype.sort argument](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Array_sort_argument) 119. [TypeError: invalid assignment to const "x"](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_const_assignment) 120. [TypeError: Iterator/AsyncIterator constructor can't be used directly](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Constructor_cant_be_used_directly) 121. [TypeError: matchAll/replaceAll must be called with a global RegExp](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Requires_global_RegExp) 122. [TypeError: More arguments needed](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/More_arguments_needed) 123. [TypeError: null/undefined has no properties](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/No_properties) 124. [TypeError: property "x" is non-configurable and can't be deleted](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Cant_delete) 125. [TypeError: Reduce of empty array with no initial value](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Reduce_of_empty_array_with_no_initial_value) 126. [TypeError: setting getter-only property "x"](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Getter_only) 127. [TypeError: WeakSet key/WeakMap value 'x' must be an object or an unregistered symbol](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Key_not_weakly_held) 128. [TypeError: X.prototype.y called on incompatible type](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Called_on_incompatible_type) 129. [URIError: malformed URI sequence](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Malformed_URI) 130. [Warning: -file- is being assigned a //\# sourceMappingURL, but already has one](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Already_has_pragma) 131. [Warning: unreachable code after return statement](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Stmt_after_return) 14. Misc 1. [JavaScript technologies overview](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/JavaScript_technologies_overview) 2. [Execution model](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Execution_model) 3. [Lexical grammar](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Lexical_grammar) 4. [Iteration protocols](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols) 5. [Strict mode](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode) 6. [Template literals](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals) 7. [Trailing commas](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Trailing_commas) 8. [Deprecated features](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Deprecated_and_obsolete_features) Your blueprint for a better internet. 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Readable Markdown	## [Description](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#description) Function values are typically instances of [`Function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function). See [`Function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function) for information on properties and methods of `Function` objects. Callable values cause [`typeof`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof) to return `"function"` instead of `"object"`. Note: Not all callable values are `instanceof Function`. For example, the `Function.prototype` object is callable but not an instance of `Function`. You can also manually set the [prototype chain](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Inheritance_and_the_prototype_chain) of your function so it no longer inherits from `Function.prototype`. However, such cases are extremely rare. ### [Return value](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#return_value) By default, if a function's execution doesn't end at a [`return`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/return) statement, or if the `return` keyword doesn't have an expression after it, then the return value is [`undefined`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined). The `return` statement allows you to return an arbitrary value from the function. One function call can only return one value, but you can simulate the effect of returning multiple values by returning an object or array and [destructuring](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring) the result. Note: Constructors called with [`new`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/new) have a different set of logic to determine their return values. ### [Passing arguments](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#passing_arguments) [Parameters and arguments](https://en.wikipedia.org/wiki/Parameter_$computer_programming$#Parameters_and_arguments) have slightly different meanings, but in MDN web docs, we often use them interchangeably. For a quick reference: In this example, the `num` variable is called the function's parameter: it's declared in the parenthesis-enclosed list of the function's definition. The function expects the `num` parameter to be a number — although this is not enforceable in JavaScript without writing runtime validation code. In the `formatNumber(2)` call, the number `2` is the function's argument: it's the value that is actually passed to the function in the function call. The argument value can be accessed inside the function body through the corresponding parameter name or the [`arguments`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments) object. Arguments are always [passed by value](https://en.wikipedia.org/wiki/Evaluation_strategy#Call_by_reference) and never passed by reference. This means that if a function reassigns a parameter, the value won't change outside the function. More precisely, object arguments are [passed by sharing](https://en.wikipedia.org/wiki/Evaluation_strategy#Call_by_sharing), which means if the object's properties are mutated, the change will impact the outside of the function. For example: The [`this`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this) keyword refers to the object that the function is accessed on — it does not refer to the currently executing function, so you must refer to the function value by name, even within the function body. ### [Defining functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#defining_functions) Broadly speaking, JavaScript has four kinds of functions: - Regular function: can return anything; always runs to completion after invocation - Generator function: returns a [`Generator`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Generator) object; can be paused and resumed with the [`yield`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/yield) operator - Async function: returns a [`Promise`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise); can be paused and resumed with the [`await`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await) operator - Async generator function: returns an [`AsyncGenerator`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncGenerator) object; both the `await` and `yield` operators can be used For every kind of function, there are multiple ways to define it: [Declaration](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#declaration) [`function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function), [`function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function), [`async function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function), [`async function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function) [Expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#expression) [`function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/function), [`function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/function), [`async function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/async_function), [`async function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/async_function) [Constructor](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#constructor) [`Function()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/Function), [`GeneratorFunction()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/GeneratorFunction/GeneratorFunction), [`AsyncFunction()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncFunction/AsyncFunction), [`AsyncGeneratorFunction()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncGeneratorFunction/AsyncGeneratorFunction) In addition, there are special syntaxes for defining [arrow functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions) and [methods](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Method_definitions), which provide more precise semantics for their usage. [Classes](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes) are conceptually not functions (because they throw an error when called without `new`), but they also inherit from `Function.prototype` and have `typeof MyClass === "function"`. All syntaxes do approximately the same thing, but there are some subtle behavior differences. - The `Function()` constructor, `function` expression, and `function` declaration syntaxes create full-fledged function objects, which can be constructed with [`new`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/new). However, arrow functions and methods cannot be constructed. Async functions, generator functions, and async generator functions are not constructible regardless of syntax. - The `function` declaration creates functions that are [hoisted](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions#function_hoisting). Other syntaxes do not hoist the function and the function value is only visible after the definition. - The arrow function and `Function()` constructor always create anonymous functions, which means they can't easily call themselves recursively. One way to call an arrow function recursively is by assigning it to a variable. - The arrow function syntax does not have access to `arguments` or `this`. - The `Function()` constructor cannot access any local variables — it only has access to the global scope. - The `Function()` constructor causes runtime compilation and is often slower than other syntaxes. For `function` expressions, there is a distinction between the function name and the variable the function is assigned to. The function name cannot be changed, while the variable the function is assigned to can be reassigned. The function name can be different from the variable the function is assigned to — they have no relation to each other. The function name can be used only within the function's body. Attempting to use it outside the function's body results in an error (or gets another value, if the same name is declared elsewhere). For example: On the other hand, the variable the function is assigned to is limited only by its scope, which is guaranteed to include the scope in which the function is declared. A function declaration also creates a variable with the same name as the function name. Thus, unlike those defined by function expressions, functions defined by function declarations can be accessed by their name in the scope they were defined in, as well as in their own body. A function defined by `new Function` will dynamically have its source assembled, which is observable when you serialize it. For example, `console.log(new Function().toString())` gives: This is the actual source used to compile the function. However, although the `Function()` constructor will create the function with name `anonymous`, this name is not added to the scope of the body. The body only ever has access to global variables. For example, the following would result in an error: A function defined by a function expression or by a function declaration inherits the current scope. That is, the function forms a closure. On the other hand, a function defined by a `Function` constructor does not inherit any scope other than the global scope (which all functions inherit). Functions defined by function expressions and function declarations are parsed only once, while a function defined by the `Function` constructor parses the string passed to it each and every time the constructor is called. Although a function expression creates a closure every time, the function body is not reparsed, so function expressions are still faster than `new Function(...)`. Therefore the `Function` constructor should generally be avoided whenever possible. A function declaration may be unintentionally turned into a function expression when it appears in an expression context. On the other hand, a function expression may also be turned into a function declaration. An [expression statement](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/Expression_statement) cannot begin with the `function` or `async function` keywords, which is a common mistake when implementing [IIFEs](https://developer.mozilla.org/en-US/docs/Glossary/IIFE) (Immediately Invoked Function Expressions). Instead, start the expression statement with something else, so that the `function` keyword unambiguously starts a function expression. Common options include [grouping](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Grouping) and using [`void`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/void). ### [Function parameters](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#function_parameters) Each function parameter is a simple identifier that you can access in the local scope. There are three special parameter syntaxes: - [Default parameters](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Default_parameters) allow formal parameters to be initialized with default values if no value or `undefined` is passed. - The [rest parameter](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/rest_parameters) allows representing an indefinite number of arguments as an array. - [Destructuring](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring) allows unpacking elements from arrays, or properties from objects, into distinct variables. There are some consequences if one of the above non-simple parameter syntaxes is used: - You cannot apply `"use strict"` to the function body — this causes a [syntax error](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Strict_non_simple_params). - Even if the function is not in [strict mode](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode), certain strict mode function features apply, including that the [`arguments`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments) object stops syncing with the named parameters, [`arguments.callee`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments/callee) throws an error when accessed, and duplicate parameter names are not allowed. ### [The arguments object](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#the_arguments_object) You can refer to a function's arguments within the function by using the [`arguments`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments) object. [`arguments`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments) An array-like object containing the arguments passed to the currently executing function. [`arguments.callee`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments/callee) The currently executing function. [`arguments.length`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments/length) The number of arguments passed to the function. ### [Getter and setter functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#getter_and_setter_functions) You can define accessor properties on any standard built-in object or user-defined object that supports the addition of new properties. Within [object literals](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Object_initializer) and [classes](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes), you can use special syntaxes to define the getter and setter of an accessor property. [get](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/get) Binds an object property to a function that will be called when that property is looked up. [set](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/set) Binds an object property to a function to be called when there is an attempt to set that property. Note that these syntaxes create an object property, not a method. The getter and setter functions themselves can only be accessed using reflective APIs such as [`Object.getOwnPropertyDescriptor()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/getOwnPropertyDescriptor). ### [Block-level functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#block-level_functions) In [strict mode](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode), functions inside blocks are scoped to that block. Prior to ES2015, block-level functions were forbidden in strict mode. ### [Block-level functions in non-strict code](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#block-level_functions_in_non-strict_code) In a word: Don't. In non-strict code, function declarations inside blocks behave strangely. For example: The semantics of this in strict mode are well-specified — `zero` only ever exists within that scope of the `if` block. If `shouldDefineZero` is false, then `zero` should never be defined, since the block never executes. However, historically, this was left unspecified, so different browsers implemented it differently in non-strict mode. For more information, see the [`function` declaration](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function#block-level_function_declaration) reference. A safer way to define functions conditionally is to assign a function expression to a variable: ## [Examples](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#examples) ### [Returning a formatted number](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#returning_a_formatted_number) The following function returns a string containing the formatted representation of a number padded with leading zeros. The following statements call the `padZeros` function. ### [Determining whether a function exists](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#determining_whether_a_function_exists) You can determine whether a function exists by using the [`typeof`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof) operator. In the following example, a test is performed to determine if the `window` object has a property called `noFunc` that is a function. If so, it is used; otherwise, some other action is taken. Note that in the `if` test, a reference to `noFunc` is used — there are no parentheses `()` after the function name so the actual function is not called. ## [Specifications](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#specifications) \| Specification \| \|---\| \| [ECMAScript® 2027 Language Specification \# sec-function-definitions](https://tc39.es/ecma262/multipage/ecmascript-language-functions-and-classes.html#sec-function-definitions) \| ## [Browser compatibility](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#browser_compatibility) ## [See also](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#see_also) - [Functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions) guide - [Classes](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes) - [`function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function) - [`function` expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/function) - [`Function`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function)
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