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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
:
The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.
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.
Block Scoping Rules
With var
Variables declared with var
do not have block scope. Variables introduced with 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. Although "standalone" blocks are valid syntax, you do not want to use standalone blocks in JavaScript, because they don't do what you think they do, if you think they do anything like such blocks in C or Java. 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 C or Java, the equivalent code would have outputted 1.
With let
and const
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.
With function
A function declaration is also limited in scope inside the block where the declaration occurs:
foo('outside'); // TypeError: foo is not a function { function foo(location) { console.log('foo is called ' + location); } foo('inside'); // works correctly and logs 'foo is called inside' }
To be more precise, the block statement is preventing the function declaration from being hoisted to the top of the scope. The function is behaving as if it were defined as a function expression and, as such, it is only the implicit variable declaration that gets hoisted to the top of the scope:
foo; // returns undefined { function foo(location) { console.log('foo is called ' + location); } foo('inside'); // works correctly and logs 'foo is called inside' }
Consequently, this means that if we move the call to foo
below the block statement that there will be no error:
{ function foo(location) { console.log('foo is called ' + location); } foo('inside'); // works correctly and logs 'foo is called inside' } foo('outside'); // works correctly and logs 'foo is called outside'
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 | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Basic support | Chrome Full support Yes | Edge Full support Yes | Firefox Full support 1 | IE Full support Yes | Opera Full support Yes | Safari Full support Yes | WebView Android Full support Yes | Chrome Android Full support Yes | Edge Mobile Full support Yes | Firefox Android Full support 4 | Opera Android Full support Yes | Safari iOS Full support Yes | Samsung Internet Android Full support Yes | nodejs Full support Yes |
Legend
- Full support
- Full support