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
:
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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 forbreak
.
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 let
, const
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
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
block | Chrome Full support 1 | Edge Full support 12 | Firefox Full support 1 | IE Full support 11 | Opera Full support Yes | Safari Full support Yes | WebView Android Full support 1 | Chrome Android Full support 18 | Firefox Android Full support 4 | Opera Android Full support Yes | Safari iOS Full support Yes | Samsung Internet Android Full support 1.0 | nodejs Full support Yes |
Legend
- Full support
- Full support