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• 描述
• 构造函数
• 实例方法
• 范例
• 规范
• 浏览器兼容性
• 另请参阅
• 相关话题

WeakMap object is a collection of key/value pairs in which the keys are weakly referenced. The keys must be objects and the values can be arbitrary values.

You can learn more about WeakMap s in the WeakMap object guide (under 键控集合 ).

描述

Keys of WeakMaps are of the type Object only. Primitive data types as keys are not allowed (e.g. a Symbol can't be a WeakMap key).

Why Weak Map?

A map API could be implemented in JavaScript with two arrays (one for keys, one for values) shared by the four API methods. Setting elements on this map would involve pushing a key and value onto the end of each of those arrays simultaneously. As a result, the indices of the key and value would correspond to both arrays. Getting values from the map would involve iterating through all keys to find a match, then using the index of this match to retrieve the corresponding value from the array of values.

Such an implementation would have two main inconveniences:

1. The first one is an O( n ) set and search ( n being the number of keys in the map) since both operations must iterate through the list of keys to find a matching value.
2. The second inconvenience is a memory leak because the arrays ensure that references to each key and each value are maintained indefinitely. These references prevent the keys from being garbage collected, even if there are no other references to the object. This would also prevent the corresponding values from being garbage collected.

By contrast, native WeakMap s hold "weak" references to key objects, which means that they do not prevent garbage collection in case there would be no other reference to the key object. This also avoids preventing garbage collection of values in the map. Native WeakMaps can be particularly useful constructs when mapping keys to information about the key that is valuable only if the key has not been garbage collected.

Because the references are weak, WeakMap keys are not enumerable. There is no method to obtain a list of the keys. If they were, the list would depend on the state of garbage collection, introducing non-determinism. If you want to have a list of keys, you should use a Map .

构造函数

WeakMap()

创建新的 WeakMap 对象。

实例方法

WeakMap.prototype.delete( key )

Removes any value associated to the key . WeakMap.prototype.has( key ) will return false afterwards.

WeakMap.prototype.get( key )

Returns the value associated to the key ，或 undefined 若没有。

WeakMap.prototype.has( key )

Returns a Boolean asserting whether a value has been associated to the key 在 WeakMap object or not.

WeakMap.prototype.set( key , value )

设置 value 为 key 在 WeakMap object. Returns the WeakMap 对象。

范例

Using WeakMap

const wm1 = new WeakMap(),
      wm2 = new WeakMap(),
      wm3 = new WeakMap();
const o1 = {},
      o2 = function() {},
      o3 = window;
wm1.set(o1, 37);
wm1.set(o2, 'azerty');
wm2.set(o1, o2); // a value can be anything, including an object or a function
wm2.set(o3, undefined);
wm2.set(wm1, wm2); // keys and values can be any objects. Even WeakMaps!
wm1.get(o2); // "azerty"
wm2.get(o2); // undefined, because there is no key for o2 on wm2
wm2.get(o3); // undefined, because that is the set value
wm1.has(o2); // true
wm2.has(o2); // false
wm2.has(o3); // true (even if the value itself is 'undefined')
wm3.set(o1, 37);
wm3.get(o1); // 37
wm1.has(o1); // true
wm1.delete(o1);
wm1.has(o1); // false
					

Implementing a WeakMap-like class with a .clear() method

class ClearableWeakMap {
  constructor(init) {
    this._wm = new WeakMap(init);
  }
  clear() {
    this._wm = new WeakMap();
  }
  delete(k) {
    return this._wm.delete(k);
  }
  get(k) {
    return this._wm.get(k);
  }
  has(k) {
    return this._wm.has(k);
  }
  set(k, v) {
    this._wm.set(k, v);
    return this;
  }
}
					

规范

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

浏览器兼容性

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request. 更新 GitHub 上的兼容性数据

	Desktop						Mobile						Server
	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
WeakMap	Chrome 36	Edge 12	Firefox 6	IE 11	Opera 23	Safari 8	WebView Android 37	Chrome Android 36	Firefox Android 6	Opera Android 24	Safari iOS 8	Samsung Internet Android 3.0	nodejs 0.12 0.12 0.10 Disabled Disabled From version 0.10: this feature is behind the --harmony runtime flag.
WeakMap() 构造函数	Chrome 36	Edge 12	Firefox 6	IE 11	Opera 23	Safari 8	WebView Android 37	Chrome Android 36	Firefox Android 6	Opera Android 24	Safari iOS 8	Samsung Internet Android 3.0	nodejs 0.12 0.12 0.10 Disabled Disabled From version 0.10: this feature is behind the --harmony runtime flag.
clear 弃用 非标	Chrome 36 — 43	Edge No	Firefox 20 — 46	IE 11	Opera 25 — 30	Safari 8 — 9	WebView Android 37 — 43	Chrome Android 36 — 43	Firefox Android 20 — 46	Opera Android 25 — 30	Safari iOS 8 — 9	Samsung Internet Android 3.0 — 4.0	nodejs 0.12 — 4.0.0
delete	Chrome 36	Edge 12	Firefox 6 6 Prior to Firefox 38, this method threw a TypeError when the key parameter was not an object. This has been fixed in version 38 and later to return false as per the ES2015 standard.	IE 11	Opera 23	Safari 8	WebView Android 37	Chrome Android 36	Firefox Android 6 6 Prior to Firefox 38, this method threw a TypeError when the key parameter was not an object. This has been fixed in version 38 and later to return false as per the ES2015 standard.	Opera Android 24	Safari iOS 8	Samsung Internet Android 3.0	nodejs 0.12 0.12 0.10 Disabled Disabled From version 0.10: this feature is behind the --harmony runtime flag.
get	Chrome 36	Edge 12	Firefox 6 6 Prior to Firefox 38, this method threw a TypeError when the key parameter was not an object. However, the ES2015 specification specifies to return undefined instead. Furthermore, WeakMap.prototype.get accepted an optional second argument as a fallback value, which is not part of the standard. Both non-standard behaviors are removed in version 38 and higher.	IE 11	Opera 23	Safari 8	WebView Android 37	Chrome Android 36	Firefox Android 6 6 Prior to Firefox 38, this method threw a TypeError when the key parameter was not an object. However, the ES2015 specification specifies to return undefined instead. Furthermore, WeakMap.prototype.get accepted an optional second argument as a fallback value, which is not part of the standard. Both non-standard behaviors are removed in version 38 and higher.	Opera Android 24	Safari iOS 8	Samsung Internet Android 3.0	nodejs 0.12 0.12 0.10 Disabled Disabled From version 0.10: this feature is behind the --harmony runtime flag.
has	Chrome 36	Edge 12	Firefox 6 6 Prior to Firefox 38, this method threw a TypeError when the key parameter was not an object. This has been fixed in version 38 and later to return false as per the ES2015 standard.	IE 11	Opera 23	Safari 8	WebView Android 37	Chrome Android 36	Firefox Android 6 6 Prior to Firefox 38, this method threw a TypeError when the key parameter was not an object. This has been fixed in version 38 and later to return false as per the ES2015 standard.	Opera Android 24	Safari iOS 8	Samsung Internet Android 3.0	nodejs 0.12 0.12 0.10 Disabled Disabled From version 0.10: this feature is behind the --harmony runtime flag.
set	Chrome 36	Edge 12	Firefox 6 6 Prior to Firefox 38, this method threw a TypeError when the key parameter was not an object. This has been fixed in version 38 and later to return false as per the ES2015 standard.	IE 部分支持 11 部分支持 11 Returns 'undefined' instead of the 'Map' object.	Opera 23	Safari 8	WebView Android 37	Chrome Android 36	Firefox Android 6 6 Prior to Firefox 38, this method threw a TypeError when the key parameter was not an object. This has been fixed in version 38 and later to return false as per the ES2015 standard.	Opera Android 24	Safari iOS 8	Samsung Internet Android 3.0	nodejs 0.12 0.12 0.10 Disabled Disabled From version 0.10: this feature is behind the --harmony runtime flag.

图例

完整支持

完整支持

部分支持

部分支持

不支持

不支持

非标。预期跨浏览器支持较差。

非标。预期跨浏览器支持较差。

弃用。不要用于新网站。

弃用。不要用于新网站。

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

• WeakMap in the JavaScript guide
• Hiding Implementation Details with ECMAScript 6 WeakMaps
• Map
• Set
• WeakSet

元数据

• 最后修改： Aug 19, 2020

相关话题

1. 标准内置对象
2. WeakMap
3. 方法
   1. WeakMap.prototype.clear()
   2. WeakMap.prototype.delete()
   3. WeakMap.prototype.get()
   4. WeakMap.prototype.has()
   5. WeakMap.prototype.set()
4. 继承：
5. Function
6. 特性
   1. Function.arguments
   2. Function.caller
   3. Function.displayName
   4. Function.length
   5. Function.name
7. 方法
   1. Function.prototype.apply()
   2. Function.prototype.bind()
   3. Function.prototype.call()
   4. Function.prototype.toSource()
   5. Function.prototype.toString()
8. Object
9. 特性
   1. Object.prototype.__proto__
   2. Object.prototype.constructor
10. 方法
    1. Object.prototype.__defineGetter__()
    2. Object.prototype.__defineSetter__()
    3. Object.prototype.__lookupGetter__()
    4. Object.prototype.__lookupSetter__()
    5. Object.prototype.hasOwnProperty()
    6. Object.prototype.isPrototypeOf()
    7. Object.prototype.propertyIsEnumerable()
    8. Object.prototype.toLocaleString()
    9. Object.prototype.toSource()
    10. Object.prototype.toString()
    11. Object.prototype.valueOf()
    12. Object.setPrototypeOf()