Destructuring assignment

析构赋值 syntax is a JavaScript expression that makes it possible to unpack values from arrays, or properties from objects, into distinct variables.

句法

let a, b, rest;
[a, b] = [10, 20];
console.log(a); // 10
console.log(b); // 20
[a, b, ...rest] = [10, 20, 30, 40, 50];
console.log(a); // 10
console.log(b); // 20
console.log(rest); // [30, 40, 50]
({ a, b } = { a: 10, b: 20 });
console.log(a); // 10
console.log(b); // 20
// Stage 4(finished) proposal
({a, b, ...rest} = {a: 10, b: 20, c: 30, d: 40});
console.log(a); // 10
console.log(b); // 20
console.log(rest); // {c: 30, d: 40}
					

描述

The object and array literal expressions provide an easy way to create ad hoc packages of data.

const x = [1, 2, 3, 4, 5];
					

The destructuring assignment uses similar syntax, but on the left-hand side of the assignment to define what values to unpack from the sourced variable.

const x = [1, 2, 3, 4, 5];
const [y, z] = x;
console.log(y); // 1
console.log(z); // 2
					

This capability is similar to features present in languages such as Perl and Python.

范例

Array destructuring

Basic variable assignment

const foo = ['one', 'two', 'three'];
const [red, yellow, green] = foo;
console.log(red); // "one"
console.log(yellow); // "two"
console.log(green); // "three"
					

Assignment separate from declaration

A variable can be assigned its value via destructuring separate from the variable's declaration.

let a, b;
[a, b] = [1, 2];
console.log(a); // 1
console.log(b); // 2
					

Default values

A variable can be assigned a default, in the case that the value unpacked from the array is undefined .

let a, b;
[a=5, b=7] = [1];
console.log(a); // 1
console.log(b); // 7
					

Swapping variables

Two variables values can be swapped in one destructuring expression.

Without destructuring assignment, swapping two values requires a temporary variable (or, in some low-level languages, the XOR-swap trick ).

let a = 1;
let b = 3;
[a, b] = [b, a];
console.log(a); // 3
console.log(b); // 1
const arr = [1,2,3];
[arr[2], arr[1]] = [arr[1], arr[2]];
console.log(arr); // [1,3,2]
					

Parsing an array returned from a function

It's always been possible to return an array from a function. Destructuring can make working with an array return value more concise.

在此范例中, f() returns the values [1, 2] as its output, which can be parsed in a single line with destructuring.

function f() {
  return [1, 2];
}
let a, b;
[a, b] = f();
console.log(a); // 1
console.log(b); // 2
					

Ignoring some returned values

You can ignore return values that you're not interested in:

function f() {
  return [1, 2, 3];
}
const [a, , b] = f();
console.log(a); // 1
console.log(b); // 3
const [c] = f();
console.log(c); // 1
					

You can also ignore all returned values:

[,,] = f();
					

Assigning the rest of an array to a variable

When destructuring an array, you can unpack and assign the remaining part of it to a variable using the rest pattern:

const [a, ...b] = [1, 2, 3];
console.log(a); // 1
console.log(b); // [2, 3]
					

Be aware that a SyntaxError will be thrown if a trailing comma is used on the left-hand side with a rest element:

const [a, ...b,] = [1, 2, 3];
// SyntaxError: rest element may not have a trailing comma
// Always consider using rest operator as the last element
					

Unpacking values from a regular expression match

When the regular expression exec() method finds a match, it returns an array containing first the entire matched portion of the string and then the portions of the string that matched each parenthesized group in the regular expression. Destructuring assignment allows you to unpack the parts out of this array easily, ignoring the full match if it is not needed.

function parseProtocol(url) {
  const parsedURL = /^(\w+)\:\/\/([^\/]+)\/(.*)$/.exec(url);
  if (!parsedURL) {
    return false;
  }
  console.log(parsedURL);
  // ["https://developer.mozilla.org/en-US/Web/JavaScript",
      "https", "developer.mozilla.org", "en-US/Web/JavaScript"]
  const [, protocol, fullhost, fullpath] = parsedURL;
  return protocol;
}
console.log(parseProtocol('https://developer.mozilla.org/en-US/Web/JavaScript'));
// "https"
					

Object destructuring

Basic assignment

const user = {
    id: 42,
    is_verified: true
};
const {id, is_verified} = user;
console.log(id); // 42
console.log(is_verified); // true
					

Assignment without declaration

A variable can be assigned its value with destructuring separate from its declaration.

let a, b;
({a, b} = {a: 1, b: 2});
					

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

{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.

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

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

Assigning to new variable names

A property can be unpacked from an object and assigned to a variable with a different name than the object property.

const o = {p: 42, q: true};
const {p: foo, q: bar} = o;
console.log(foo); // 42
console.log(bar); // true
					

Here, for example, const {p: foo} = o takes from the object o the property named p and assigns it to a local variable named foo .

Default values

A variable can be assigned a default, in the case that the value unpacked from the object is undefined .

const {a = 10, b = 5} = {a: 3};
console.log(a); // 3
console.log(b); // 5
					

Assigning to new variables names and providing default values

A property can be both 1) unpacked from an object and assigned to a variable with a different name and 2) assigned a default value in case the unpacked value is undefined .

const {a: aa = 10, b: bb = 5} = {a: 3};
console.log(aa); // 3
console.log(bb); // 5
					

Unpacking fields from objects passed as function parameter

const user = {
  id: 42,
  displayName: 'jdoe',
  fullName: {
    firstName: 'John',
    lastName: 'Doe'
  }
};
function userId({id}) {
  return id;
}
function whois({displayName, fullName: {firstName: name}}) {
  return `${displayName} is ${name}`;
}
console.log(userId(user)); // 42
console.log(whois(user));  // "jdoe is John"
					

This unpacks the id , displayName and firstName from the user object and prints them.

Setting a function parameter's default value

function drawChart({size = 'big', coords = {x: 0, y: 0}, radius = 25} = {}) {
  console.log(size, coords, radius);
  // do some chart drawing
}
drawChart({
  coords: {x: 18, y: 30},
  radius: 30
});
					

In the function signature for drawChart above, the destructured left-hand side is assigned to an empty object literal on the right-hand side: {size = 'big', coords = {x: 0, y: 0}, radius = 25} = {} . You could have also written the function without the right-hand side assignment. However, if you leave out the right-hand side assignment, the function will look for at least one argument to be supplied when invoked, whereas in its current form, you can simply call drawChart() without supplying any parameters. The current design is useful if you want to be able to call the function without supplying any parameters, the other can be useful when you want to ensure an object is passed to the function.

Nested object and array destructuring

const metadata = {
  title: 'Scratchpad',
  translations: [
    {
      locale: 'de',
      localization_tags: [],
      last_edit: '2014-04-14T08:43:37',
      url: '/de/docs/Tools/Scratchpad',
      title: 'JavaScript-Umgebung'
    }
  ],
  url: '/en-US/docs/Tools/Scratchpad'
};
let {
  title: englishTitle, // rename
  translations: [
    {
       title: localeTitle, // rename
    },
  ],
} = metadata;
console.log(englishTitle); // "Scratchpad"
console.log(localeTitle);  // "JavaScript-Umgebung"
					

For of iteration and destructuring

const people = [
  {
    name: 'Mike Smith',
    family: {
      mother: 'Jane Smith',
      father: 'Harry Smith',
      sister: 'Samantha Smith'
    },
    age: 35
  },
  {
    name: 'Tom Jones',
    family: {
      mother: 'Norah Jones',
      father: 'Richard Jones',
      brother: 'Howard Jones'
    },
    age: 25
  }
];
for (const {name: n, family: {father: f}} of people) {
  console.log('Name: ' + n + ', Father: ' + f);
}
// "Name: Mike Smith, Father: Harry Smith"
// "Name: Tom Jones, Father: Richard Jones"
					

Computed object property names and destructuring

Computed property names, like on object literals , can be used with destructuring.

let key = 'z';
let {[key]: foo} = {z: 'bar'};
console.log(foo); // "bar"
					

Rest in Object Destructuring

Rest/Spread Properties for ECMAScript proposal (stage 4) adds the rest syntax to destructuring. Rest properties collect the remaining own enumerable property keys that are not already picked off by the destructuring pattern.

let {a, b, ...rest} = {a: 10, b: 20, c: 30, d: 40}
a; // 10
b; // 20
rest; // { c: 30, d: 40 }
					

Invalid JavaScript identifier as a property name

Destructuring can be used with property names that are not valid JavaScript identifiers by providing an alternative identifier that is valid.

const foo = { 'fizz-buzz': true };
const { 'fizz-buzz': fizzBuzz } = foo;
console.log(fizzBuzz); // "true"
					

Combined Array and Object Destructuring

Array and Object destructuring can be combined. Say you want the third element in the array props below, and then you want the name property in the object, you can do the following:

const props = [
  { id: 1, name: 'Fizz'},
  { id: 2, name: 'Buzz'},
  { id: 3, name: 'FizzBuzz'}
];
const [,, { name }] = props;
console.log(name); // "FizzBuzz"
					

The prototype chain is looked up when the object is deconstructed

When deconstructing an object, if a property is not accessed in itself, it will continue to look up along the prototype chain.

let obj = {self: '123'};
obj.__proto__.prot = '456';
const {self, prot} = obj;
// self "123"
// prot "456"(Access to the prototype chain)
					

规范

规范
ECMAScript (ECMA-262)
The definition of 'Destructuring assignment' in that specification.

浏览器兼容性

更新 GitHub 上的兼容性数据
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Chrome Edge Firefox Internet Explorer Opera Safari Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet Node.js
Destructuring assignment Chrome 49 Edge 14 Firefox 41
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Firefox provided a non-standard destructuring implementation from Firefox 2 to 40.
IE No Opera 36 Safari 8 WebView Android 49 Chrome Android 49 Firefox Android 41
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Firefox provided a non-standard destructuring implementation from Firefox 2 to 40.
Opera Android 36 Safari iOS 8 Samsung Internet Android 5.0 nodejs 6.0.0
Computed property names Chrome 49 Edge 14 Firefox 41 IE No Opera 36 Safari 10 WebView Android 49 Chrome Android 49 Firefox Android 41 Opera Android 36 Safari iOS 10 Samsung Internet Android 5.0 nodejs 6.0.0
Rest in arrays Chrome 49 Edge 14
Disabled
14
Disabled
Disabled From version 14: this feature is behind the Enable experimental Javascript features preference.
Firefox 41 IE No Opera 36 Safari 9.1 WebView Android 49 Chrome Android 49 Firefox Android 41 Opera Android 36 Safari iOS 9.3 Samsung Internet Android 5.0 nodejs 6.0.0
Rest in objects
Chrome 60 Edge 79 Firefox 55 IE No Opera 47 Safari 11.1 WebView Android 60 Chrome Android 60 Firefox Android 55 Opera Android 44 Safari iOS 11.3 Samsung Internet Android 8.0 nodejs 8.3.0

图例

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不支持
不支持
实验。期望将来行为有所改变。
实验。期望将来行为有所改变。
见实现注意事项。
用户必须明确启用此特征。
用户必须明确启用此特征。

另请参阅

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