没有 var、let 或 const 的对象解构 [英] Object destructuring without var, let or const

问题描述

为什么对象解构前没有var关键字会报错?

Why does object destructuring throw an error if there is no var keyword in front of it?

{a, b} = {a: 1, b: 2};

抛出 SyntaxError:预期的表达式，得到 '='

以下三个例子都没有问题

The following three examples work without problems

var {a, b} = {a: 1, b: 2};
var [c, d] = [1, 2];
    [e, f] = [1, 2];

额外问题:为什么我们不需要 var 进行数组解构?

Bonus question: Why do we not need a var for array destructuring?

我在做类似的事情时遇到了问题

I ran into the problem doing something like

function () {
  var {a, b} = objectReturningFunction();

  // Now a and b are local variables in the function, right?
  // So why can't I assign values to them?

  {a, b} = objectReturningFunction();
}

推荐答案

问题源于 {...} 运算符在 JavaScript 中具有多重含义.

The issue stems from the {...} operators having multiple meanings in JavaScript.

当 { 出现在 Statement 的开头时，它总是代表一个 block，无法分配到.如果它稍后出现在 Statement 中作为 Expression，那么它将代表一个对象.

When { appears at the start of a Statement, it'll always represent a block, which can't be assigned to. If it appears later in the Statement as an Expression, then it'll represent an Object.

var 有助于区分这一点，因为它后面不能跟 Statement，分组括号:

The var helps make this distinction, since it can't be followed by a Statement, as will grouping parenthesis:

( {a, b} = objectReturningFunction() );

来自他们的文档:https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment#Assignment_without_declaration

注意:在没有声明的情况下使用对象字面量解构赋值时，赋值语句周围的括号 ( ... ) 是必需的.

Notes: The parentheses ( ... ) around the assignment statement are required when using object literal destructuring assignment without a declaration.

{a, b} = {a: 1, b: 2} 不是有效的独立语法，因为左侧的 {a, b} 被视为块而不是对象文字.

{a, b} = {a: 1, b: 2} is not valid stand-alone syntax, as the {a, b} on the left-hand side is considered a block and not an object literal.

然而，({a, b} = {a: 1, b: 2}) 是有效的，var {a, b} = {a: 1, b: 2} 也是如此

However, ({a, b} = {a: 1, b: 2}) is valid, as is var {a, b} = {a: 1, b: 2}

你的 ( ... ) 表达式前面需要有一个分号，否则它可以用来执行上一行的函数.

Your ( ... ) expression needs to be preceded by a semicolon or it may be used to execute a function on the previous line.

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