Web/JavaScript/Reference/Statements

JavaScript applications consist of statements with an appropriate syntax. A single statement may span multiple lines. Multiple statements may occur on a single line if each statement is separated by a semicolon. This isn't a keyword, but a group of keywords.

Statements and declarations by category

For an alphabetical listing see the sidebar on the left.

Control flow

Block: A block statement is used to group zero or more statements. The block is delimited by a pair of curly brackets.
break: Terminates the current loop, switch, or label statement and transfers program control to the statement following the terminated statement.
continue: Terminates execution of the statements in the current iteration of the current or labeled loop, and continues execution of the loop with the next iteration.
Empty: An empty statement is used to provide no statement, although the JavaScript syntax would expect one.
if...else: Executes a statement if a specified condition is true. If the condition is false, another statement can be executed.
switch: Evaluates an expression, matching the expression's value to a case clause, and executes statements associated with that case.
throw: Throws a user-defined exception.
try...catch: Marks a block of statements to try, and specifies a response, should an exception be thrown.

Declarations

var: Declares a variable, optionally initializing it to a value.
let: Declares a block scope local variable, optionally initializing it to a value.
const: Declares a read-only named constant.

Functions and classes

function: Declares a function with the specified parameters.
function*: Generator Functions enable writing iterators more easily.
async function: Declares an async function with the specified parameters.
return: Specifies the value to be returned by a function.
class: Declares a class.

Iterations

do...while: Creates a loop that executes a specified statement until the test condition evaluates to false. The condition is evaluated after executing the statement, resulting in the specified statement executing at least once.
for: Creates a loop that consists of three optional expressions, enclosed in parentheses and separated by semicolons, followed by a statement executed in the loop.
[[../../../../Archive/Web/JavaScript/for_each..|for each...in]]: Iterates a specified variable over all values of object's properties. For each distinct property, a specified statement is executed.
for...in: Iterates over the enumerable properties of an object, in arbitrary order. For each distinct property, statements can be executed.
for...of: Iterates over iterable objects (including arrays, array-like objects, iterators and generators), invoking a custom iteration hook with statements to be executed for the value of each distinct property.
for await...of: Iterates over async iterable objects, array-like objects, iterators and generators, invoking a custom iteration hook with statements to be executed for the value of each distinct property.
while: Creates a loop that executes a specified statement as long as the test condition evaluates to true. The condition is evaluated before executing the statement.

Others

debugger: Invokes any available debugging functionality. If no debugging functionality is available, this statement has no effect.
export: Used to export functions to make them available for imports in external modules, and other scripts.
import: Used to import functions exported from an external module, another script.
label: Provides a statement with an identifier that you can refer to using a break or continue statement.

with: Extends the scope chain for a statement.

Specifications

Specification
ECMAScript (ECMA-262)The definition of 'ECMAScript Language: Statements and Declarations' in that specification.

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
`async function`	Chrome Full support 55	Edge Full support 15	Firefox Full support 52	IE No support No	Opera Full support 42	Safari Full support 10.1	WebView Android Full support 55	Chrome Android Full support 55	Firefox Android Full support 52	Opera Android Full support 42	Safari iOS Full support 10.3	Samsung Internet Android Full support 6.0	nodejs Full support 7.6.0 Full support 7.6.0 Full support 7.0.0 Disabled' Disabled' From version 7.0.0: this feature is behind the `--harmony` runtime flag.
`block`	Chrome Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support 11	Opera Full support 3	Safari Full support 1	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 1	Samsung Internet Android Full support 1.0	nodejs Full support 0.1.100
`break`	Chrome Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support 3	Opera Full support 4	Safari Full support 1	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 1	Samsung Internet Android Full support 1.0	nodejs Full support 0.1.100
`class`	Chrome Full support 49 Full support 49 No support 42 — 49 Notes' Notes' Strict mode is required. No support 42 — 49 Disabled' Disabled' From version 42 until version 49 (exclusive): this feature is behind the `Experimental JavaScript` preference (needs to be set to `Enabled`). To change preferences in Chrome, visit chrome://flags.	Edge Full support 13	Firefox Full support 45	IE No support No	Opera Full support 36 Full support 36 No support 29 — 36 Notes' Notes' Strict mode is required. No support 29 — 36 Disabled' Disabled' From version 29 until version 36 (exclusive): this feature is behind the `Experimental JavaScript` preference (needs to be set to `Enabled`).	Safari Full support 10.1	WebView Android Full support 49 Full support 49 No support 42 — 49 Notes' Notes' Strict mode is required.	Chrome Android Full support 49 Full support 49 No support 42 — 49 Notes' Notes' Strict mode is required. No support 42 — 49 Disabled' Disabled' From version 42 until version 49 (exclusive): this feature is behind the `Experimental JavaScript` preference (needs to be set to `Enabled`). To change preferences in Chrome, visit chrome://flags.	Firefox Android Full support 45	Opera Android Full support 36 Full support 36 No support 29 — 36 Notes' Notes' Strict mode is required. No support 29 — 36 Disabled' Disabled' From version 29 until version 36 (exclusive): this feature is behind the `Experimental JavaScript` preference (needs to be set to `Enabled`).	Safari iOS Full support 10.3	Samsung Internet Android Full support 5.0 Full support 5.0 No support 4.0 — 5.0 Notes' Notes' Strict mode is required.	nodejs Full support 6.0.0
`const`	Chrome Full support 21	Edge Full support 12	Firefox Full support 36 Notes' Full support 36 Notes' Notes' Prior to Firefox 13, `const` is implemented, but re-assignment is not failing. Notes' Prior to Firefox 46, a `TypeError` was thrown on redeclaration instead of a `SyntaxError`.	IE Full support 11	Opera Full support 9	Safari Full support 5.1	WebView Android Full support ≤37	Chrome Android Full support 25	Firefox Android Full support 36 Notes' Full support 36 Notes' Notes' Prior to Firefox 13, `const` is implemented, but re-assignment is not failing. Notes' Prior to Firefox 46, a `TypeError` was thrown on redeclaration instead of a `SyntaxError`.	Opera Android Full support 10.1	Safari iOS Full support 6	Samsung Internet Android Full support 1.5	nodejs Full support 6.0.0
`continue`	Chrome Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support 3	Opera Full support 4	Safari Full support 1	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 1	Samsung Internet Android Full support 1.0	nodejs Full support 0.1.100
`debugger`	Chrome Full support 5	Edge Full support 12	Firefox Full support 1	IE Full support 4	Opera Full support 10	Safari Full support 5	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 4.2	Samsung Internet Android Full support 1.0	nodejs Full support 0.1.100
`do...while`	Chrome Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support 4	Opera Full support 4	Safari Full support 1	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 1	Samsung Internet Android Full support 1.0	nodejs Full support 0.1.100
Empty statement (`;`)	Chrome Full support 3	Edge Full support 12	Firefox Full support 1	IE Full support 3	Opera Full support 3	Safari Full support 5	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 4.2	Samsung Internet Android Full support 1.0	nodejs Full support 0.1.100
`export`	Chrome Full support 61	Edge Full support 16 Full support 16 Full support 15 Disabled' Disabled' From version 15: this feature is behind the `Experimental JavaScript Features` preference.	Firefox Full support 60 Full support 60 No support 54 — 60 Disabled' Disabled' From version 54 until version 60 (exclusive): this feature is behind the `dom.moduleScripts.enabled` preference. To change preferences in Firefox, visit about:config.	IE No support No	Opera Full support 48	Safari Full support 10.1	WebView Android No support No	Chrome Android Full support 61	Firefox Android Full support 60 Full support 60 No support 54 — 60 Disabled' Disabled' From version 54 until version 60 (exclusive): this feature is behind the `dom.moduleScripts.enabled` preference. To change preferences in Firefox, visit about:config.	Opera Android Full support 45	Safari iOS Full support 10.3	Samsung Internet Android Full support 8.0	nodejs Full support 13.2.0 Notes' Full support 13.2.0 Notes' Notes' Modules must either have a filename ending in `.mjs`, or the nearest parent `package.json` file must contain `"type": "module"`. See Node's ECMAScript Modules documentation for more details. Full support 12.0.0 Notes' Disabled' Notes' Modules must either have a filename ending in `.mjs`, or the nearest parent `package.json` file must contain `"type": "module"`. See Node's ECMAScript Modules documentation for more details. Disabled' From version 12.0.0: this feature is behind the `--experimental-modules` runtime flag. Full support 8.5.0 Notes' Disabled' Notes' Module filenames must end with `.mjs`, not .js. See Node's ECMAScript Modules documentation for more details. Disabled' From version 8.5.0: this feature is behind the `--experimental-modules` runtime flag.
`for`	Chrome Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support 3	Opera Full support 3	Safari Full support 1	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 1	Samsung Internet Android Full support 1.0	nodejs Full support 0.1.100
`for await...of`	Chrome Full support 63	Edge Full support 79	Firefox Full support 57	IE No support No	Opera Full support 50	Safari Full support 11	WebView Android Full support 63	Chrome Android Full support 63	Firefox Android Full support 57	Opera Android Full support 46	Safari iOS Full support 11	Samsung Internet Android Full support 8.0	nodejs Full support 10.0.0 Full support 10.0.0 No support 8.10.0 — 10.0.0 Disabled' Disabled' From version 8.10.0 until version 10.0.0 (exclusive): this feature is behind the `--harmony-async-iteration` runtime flag.
`for...in`	Chrome Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support 6	Opera Full support 2	Safari Full support 1	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 1	Samsung Internet Android Full support 1.0	nodejs Full support 0.1.100
`for...of`	Chrome Full support 38	Edge Full support 12	Firefox Full support 13 Notes' Full support 13 Notes' Notes' Prior to Firefox 51, using the `for...of` loop construct with the `const` keyword threw a `SyntaxError` ("missing = in const declaration").	IE No support No	Opera Full support 25	Safari Full support 7	WebView Android Full support 38	Chrome Android Full support 38	Firefox Android Full support 14 Notes' Full support 14 Notes' Notes' Prior to Firefox 51, using the `for...of` loop construct with the `const` keyword threw a `SyntaxError` ("missing = in const declaration").	Opera Android Full support 25	Safari iOS Full support 7	Samsung Internet Android Full support 3.0	nodejs Full support 0.12
`function`	Chrome Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support 3	Opera Full support 3	Safari Full support 1	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 1	Samsung Internet Android Full support 1.0	nodejs Full support 0.1.100
`function*`	Chrome Full support 39	Edge Full support 13	Firefox Full support 26	IE No support No	Opera Full support 26	Safari Full support 10	WebView Android Full support 39	Chrome Android Full support 39	Firefox Android Full support 26	Opera Android Full support 26	Safari iOS Full support 10	Samsung Internet Android Full support 4.0	nodejs Full support 4.0.0 Full support 4.0.0 Full support 0.12 Disabled' Disabled' From version 0.12: this feature is behind the `--harmony` runtime flag.
`if...else`	Chrome Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support 3	Opera Full support 3	Safari Full support 1	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 1	Samsung Internet Android Full support 1.0	nodejs Full support 0.1.100
`import`	Chrome Full support 61	Edge Full support 16 Full support 16 Full support 15 Disabled' Disabled' From version 15: this feature is behind the `Experimental JavaScript Features` preference.	Firefox Full support 60 Full support 60 No support 54 — 60 Disabled' Disabled' From version 54 until version 60 (exclusive): this feature is behind the `dom.moduleScripts.enabled` preference. To change preferences in Firefox, visit about:config.	IE No support No	Opera Full support 48	Safari Full support 10.1	WebView Android Full support 61	Chrome Android Full support 61	Firefox Android Full support 60 Full support 60 No support 54 — 60 Disabled' Disabled' From version 54 until version 60 (exclusive): this feature is behind the `dom.moduleScripts.enabled` preference. To change preferences in Firefox, visit about:config.	Opera Android Full support 45	Safari iOS Full support 10.3	Samsung Internet Android Full support 8.0	nodejs Full support 13.2.0 Notes' Full support 13.2.0 Notes' Notes' Modules must either have a filename ending in `.mjs`, or the nearest parent `package.json` file must contain `"type": "module"`. See Node's ECMAScript Modules documentation for more details. Full support 12.0.0 Notes' Disabled' Notes' Modules must either have a filename ending in `.mjs`, or the nearest parent `package.json` file must contain `"type": "module"`. See Node's ECMAScript Modules documentation for more details. Disabled' From version 12.0.0: this feature is behind the `--experimental-modules` runtime flag. Full support 8.5.0 Notes' Disabled' Notes' Module filenames must end with `.mjs`, not .js. See Node's ECMAScript Modules documentation for more details. Disabled' From version 8.5.0: this feature is behind the `--experimental-modules` runtime flag.
`import.meta`	Chrome Full support 64	Edge Full support 79	Firefox Full support 62	IE No support No	Opera Full support 51	Safari Full support 11.1	WebView Android Full support 64	Chrome Android Full support 64	Firefox Android Full support 62	Opera Android Full support 47	Safari iOS Full support 12	Samsung Internet Android Full support 9.0	nodejs Full support 10.4.0
`label`	Chrome Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support 4	Opera Full support 4	Safari Full support 1	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 1	Samsung Internet Android Full support 1.0	nodejs Full support 0.1.100
`let`	Chrome Full support 49 Full support 49 No support 48 — 49 Notes' Disabled' Notes' Support outside of strict mode. Disabled' From version 48 until version 49 (exclusive): this feature is behind the `Enable Experimental JavaScript Features` preference. To change preferences in Chrome, visit chrome://flags. No support 41 — 49 Notes' Notes' Strict mode is required.	Edge Full support 14 Full support 14 No support 12 — 14 Notes' Notes' In Edge 12 and 13, `let` within a `for` loop initializer does not create a separate variable for each loop iteration as defined by ES2015. Instead, it behaves as though the loop were wrapped in a scoping block with the `let` immediately before the loop.	Firefox Full support 44 Notes' Full support 44 Notes' Notes' Prior to Firefox 44, `let` is only available to code blocks in HTML wrapped in a `<script type="application/javascript;version=1.7">` block (or higher version) and has different semantics (e.g. no temporal dead zone). Notes' Prior to Firefox 46, a `TypeError` is thrown on redeclaration instead of a `SyntaxError`. Notes' Firefox 54 adds support of `let` in workers.	IE Partial support 11 Notes' Partial support 11 Notes' Notes' In Internet Explorer, `let` within a `for` loop initializer does not create a separate variable for each loop iteration as defined by ES2015. Instead, it behaves as though the loop were wrapped in a scoping block with the `let` immediately before the loop.	Opera Full support 17	Safari Full support 10	WebView Android Full support 49 Full support 49 No support 41 — 49 Notes' Notes' Strict mode is required.	Chrome Android Full support 49 Full support 49 No support 48 — 49 Notes' Disabled' Notes' Support outside of strict mode. Disabled' From version 48 until version 49 (exclusive): this feature is behind the `Enable Experimental JavaScript Features` preference. To change preferences in Chrome, visit chrome://flags. No support 41 — 49 Notes' Notes' Strict mode is required.	Firefox Android Full support 44 Notes' Full support 44 Notes' Notes' Prior to Firefox 44, `let` is only available to code blocks in HTML wrapped in a `<script type="application/javascript;version=1.7">` block (or higher version) and has different semantics (e.g. no temporal dead zone). Notes' Prior to Firefox 46, a `TypeError` is thrown on redeclaration instead of a `SyntaxError`. Notes' Firefox 54 adds support of `let` in workers.	Opera Android Full support 18	Safari iOS Full support 10	Samsung Internet Android Full support 5.0 Full support 5.0 No support 4.0 — 5.0 Notes' Notes' Strict mode is required.	nodejs Full support 6.0.0
`return`	Chrome Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support 3	Opera Full support 3	Safari Full support 1	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 1	Samsung Internet Android Full support 1.0	nodejs Full support 0.1.100
`switch`	Chrome Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support 4	Opera Full support 4	Safari Full support 1	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 1	Samsung Internet Android Full support 1.0	nodejs Full support 0.1.100
`throw`	Chrome Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support 5	Opera Full support 4	Safari Full support 1	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 1	Samsung Internet Android Full support 1.0	nodejs Full support 0.1.100
`try...catch`	Chrome Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support 5	Opera Full support 4	Safari Full support 1	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 1	Samsung Internet Android Full support 1.0	nodejs Full support 0.1.100
`var`	Chrome Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support 3	Opera Full support 3	Safari Full support 1	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 1	Samsung Internet Android Full support 1.0	nodejs Full support 0.1.100
`while`	Chrome Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support 3	Opera Full support 3	Safari Full support 1	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 1	Samsung Internet Android Full support 1.0	nodejs Full support 0.1.100
`with` Deprecated'	Chrome Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support 3	Opera Full support 4	Safari Full support 1	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 1	Samsung Internet Android Full support 1.0	nodejs Full support 0.1.100

Legend

Full support: Full support
Partial support: Partial support
No support: No support
Deprecated. Not for use in new websites.': Deprecated. Not for use in new websites.
See implementation notes.': See implementation notes.
User must explicitly enable this feature.': User must explicitly enable this feature.