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 braces ("curly brackets") and may optionally be labelled:
Syntax
Block Statement
{
StatementList
}
Labelled Block Statement
LabelIdentifier: {
StatementList
}
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...else
and for
statements.
Examples
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.
Block scoping rules 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 |
---|
ECMAScript Language Specification # sec-block |
Browser compatibility
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