Bloco (block)

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Bloco

Uma declaralção bloco (ou declaração composto em outras linguagens) é utilizada para agrupar zero ou mais declarações. O bloco é delimitado por um par de chavetas (“chavetas { }”) e opcionalmente poderá ser labelled:

Sintaxe

Declaração de Bloco

{
  StatementList
}

Declaração de Bloco Etiquetado

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

Descrição

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.

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.

Especificações

Especificação
ECMAScript (ECMA-262)
The definition of 'Block statement' in that specification.

Compatibilidade de navegador

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 3Safari Full support 1WebView Android Full support 1Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support 10.1Safari iOS Full support 1Samsung Internet Android Full support 1.0nodejs Full support 0.1.100

Legend

Full support  
Full support

Consulte também