block

A block statement (or compound statement in other languages) is used to group zero or more statements. The block is delimited by a pair of curly brackets and may optionally be labelled:

Syntax

Block Statement

{
  StatementList
}

Labelled Block Statement

LabelIdentifier: {
  StatementList
}
StatementList
Statements grouped within the block statement.
LabelIdentifier
An optional label for visual identification or as a target for break.

Description

The block statement is often called compound statement in other languages. It allows you to use multiple statements where JavaScript expects only one statement. Combining statements into blocks is a common practice in JavaScript. The opposite behavior is possible using an empty statement, where you provide no statement, although one is required.

Blocks are commonly used in association with if and for statements.

Block Scoping Rules

With var or function declaration in non-strict mode

Variables declared with var or created by function declarations in non-strict mode do not have block scope. Variables introduced within a block are scoped to the containing function or script, and the effects of setting them persist beyond the block itself. In other words, block statements do not introduce a scope. For example:

var x = 1;
{
  var x = 2;
}
console.log(x); // logs 2

This logs 2 because the var x statement within the block is in the same scope as the var x statement before the block.

In non-strict code, function declarations inside blocks behave strangely. Do not use them.

With letconst or function declaration in strict mode

By contrast, identifiers declared with let and const do have block scope:

let x = 1;
{
  let x = 2;
}
console.log(x); // logs 1

The x = 2 is limited in scope to the block in which it was defined.

The same is true of const:

const c = 1;
{
  const c = 2;
}
console.log(c); // logs 1 and does not throw SyntaxError...

Note that the block-scoped const c = 2 does not throw a SyntaxError: Identifier 'c' has already been declared because it can be declared uniquely within the block.

In strict mode, starting with ES2015, functions inside blocks are scoped to that block. Prior to ES2015, block-level functions were forbidden in strict mode. 

Specifications

Specification Status Comment
ECMAScript Latest Draft (ECMA-262)
The definition of 'Block statement' in that specification.
Draft
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Block statement' in that specification.
Standard
ECMAScript 5.1 (ECMA-262)
The definition of 'Block statement' in that specification.
Standard
ECMAScript 3rd Edition (ECMA-262)
The definition of 'Block statement' in that specification.
Standard
ECMAScript 1st Edition (ECMA-262)
The definition of 'Block statement' in that specification.
Standard Initial definition. Implemented in JavaScript 1.0.

Browser compatibility

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
blockChrome Full support 1Edge Full support 12Firefox Full support 1IE Full support 11Opera Full support YesSafari Full support YesWebView Android Full support 1Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support 1.0nodejs Full support Yes

Legend

Full support  
Full support

See also