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

return

Specifies the value to be returned by a function.

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.

throw

Throws a user-defined exception.

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.

try...catch

Marks a block of statements to try, and specifies a response, should an exception be thrown.

Declaring variables

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.

using

Declares local variables that aresynchronously disposed.

await using

Declares local variables that areasynchronously disposed.

Functions and classes

function

Declares a function with the specified parameters.

function*

Generator Functions enable writingiterators more easily.

async function

Declares an async function with the specified parameters.

async function*

Asynchronous Generator Functions enable writing asynciterators more easily.

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.

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 (includingarrays, 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

Empty

An empty statement is used to provide no statement, although the JavaScript syntax would expect one.

Block

A block statement is used to group zero or more statements. The block is delimited by a pair of curly braces.

Expression statement

An expression statement evaluates an expression and discards its result. It allows the expression to perform side effects, such as executing a function or updating a variable.

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 abreak orcontinue statement.

withDeprecated

Extends the scope chain for a statement.

Difference between statements and declarations

In this section, we will be mixing two kinds of constructs:statements anddeclarations. They are two disjoint sets of grammars. The following are declarations:

• let
• const
• function
• function*
• async function
• async function*
• class
• export (Note: it can only appear at the top-level of amodule)
• import (Note: it can only appear at the top-level of amodule)

Everything else in thelist above is a statement.

The terms "statement" and "declaration" have a precise meaning in the formal syntax of JavaScript that affects where they may be placed in code. For example, in most control-flow structures, the body only accepts statements — such as the two arms of anif...else:

if (condition)  statement1;else  statement2;

If you use a declaration instead of a statement, it would be aSyntaxError. For example, alet declaration is not a statement, so you can't use it in its bare form as the body of anif statement.

if (condition)  let i = 0; // SyntaxError: Lexical declaration cannot appear in a single-statement context

On the other hand,var is a statement, so you can use it on its own as theif body.

if (condition)  var i = 0;

You can see declarations as "binding identifiers to values", and statements as "carrying out actions". The fact thatvar is a statement instead of a declaration is a special case, because it doesn't follow normal lexical scoping rules and may create side effects — in the form of creating global variables, mutating existingvar-defined variables, and defining variables that are visible outside of its block (becausevar-defined variables aren't block-scoped).

As another example,labels can only be attached to statements.

label: const a = 1; // SyntaxError: Lexical declaration cannot appear in a single-statement context

To get around this, you can wrap the declaration in braces — this makes it part of ablock statement.

label: {  const a = 1;}if (condition) {  let i = 0;}

Browser compatibility

See also

• Expressions and operators