WeakRef | JavaScript

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描述
构造函数
实例方法
Avoid where possible
Notes on WeakRefs
范例
规范
浏览器兼容性
另请参阅
相关话题

A WeakRef object lets you hold a weak reference to another object, without preventing that object from getting garbage-collected.

描述

A WeakRef object contains a weak reference to an object, which is called its target or referent . A weak reference to an object is a reference that does not prevent the object from being reclaimed by the garbage collector. In contrast, a normal (or strong ) reference keeps an object in memory. When an object no longer has any strong references to it, the JavaScript engine's garbage collector may destroy the object and reclaim its memory. If that happens, you can't get the object from a weak reference anymore.

注意： Please see the Avoid where possible section below. Correct use of WeakRef takes careful thought, and it's best avoided if possible.

构造函数

WeakRef(): 创建新的 WeakRef 对象。

实例方法

WeakRef.prototype.deref(): 返回 WeakRef object's target object, or undefined if the target object has been reclaimed.

Avoid where possible

Correct use of WeakRef takes careful thought, and it's best avoided if possible. It's also important to avoid relying on any specific behaviors not guaranteed by the specification. When, how, and whether garbage collection occurs is down to the implementation of any given JavaScript engine. Any behavior you observe in one engine may be different in another engine, in another version of the same engine, or even in a slightly different situation with the same version of the same engine. Garbage collection is a hard problem that JavaScript engine implementers are constantly refining and improving their solutions to.

Here are some specific points that the authors of the WeakRef proposal included in its explainer document :

Garbage collectors are complicated. If an application or library depends on GC cleaning up a WeakRef or calling a finalizer [cleanup callback] in a timely, predictable manner, it's likely to be disappointed: the cleanup may happen much later than expected, or not at all. Sources of variability include:

One object might be garbage-collected much sooner than another object, even if they become unreachable at the same time, e.g., due to generational collection.
Garbage collection work can be split up over time using incremental and concurrent techniques.
Various runtime heuristics can be used to balance memory usage, responsiveness.
The JavaScript engine may hold references to things which look like they are unreachable (e.g., in closures, or inline caches).
Different JavaScript engines may do these things differently, or the same engine may change its algorithms across versions.
Complex factors may lead to objects being held alive for unexpected amounts of time, such as use with certain APIs.

Notes on WeakRefs

Some notes on WeakRef s:

If your code has just created a WeakRef for a target object, or has gotten a target object from a WeakRef 's deref method, that target object will not be reclaimed until the end of the current JavaScript job (including any promise reaction jobs that run at the end of a script job). That is, you can only "see" an object get reclaimed between turns of the event loop. This is primarily to avoid making the behavior of any given JavaScript engine's garbage collector apparent in code — because if it were, people would write code relying on that behavior, which would break when the garbage collector's behavior changed. (Garbage collection is a hard problem; JavaScript engine implementers are constantly refining and improving how it works.)
If multiple WeakRef s have the same target, they're consistent with one another. The result of calling deref on one of them will match the result of calling deref on another of them (in the same job), you won't get the target object from one of them but undefined from another.
If the target of a WeakRef is also in a FinalizationRegistry ， WeakRef 's target is cleared at the same time or before any cleanup callback associated with the registry is called; if your cleanup callback calls deref on a WeakRef for the object, it will receive undefined .
You cannot change the target of a WeakRef , it will always only ever be the original target object or undefined when that target has been reclaimed.
A WeakRef might never return undefined from deref , even if nothing strongly holds the target, because the garbage collector may never decide to reclaim the object.

范例

Using a WeakRef object

This example starts a counter shown in a DOM element, stopping when the element doesn't exist anymore:

class Counter {
  constructor(element) {
    // Remember a weak reference to the DOM element
    this.ref = new WeakRef(element);
    this.start();
  }
  start() {
    if (this.timer) {
      return;
    }
    this.count = 0;
    const tick = () => {
      // Get the element from the weak reference, if it still exists
      const element = this.ref.deref();
      if (element) {
        element.textContent = ++this.count;
      } else {
        // The element doesn't exist anymore
        console.log("The element is gone.");
        this.stop();
        this.ref = null;
      }
    };
    tick();
    this.timer = setInterval(tick, 1000);
  }
  stop() {
    if (this.timer) {
      clearInterval(this.timer);
      this.timer = 0;
    }
  }
}
const counter = new Counter(document.getElementById("counter"));
counter.start();
setTimeout(() => {
  document.getElementById("counter").remove();
}, 5000);

规范

规范
WeakRefs The definition of 'WeakRef' 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
`WeakRef`	Chrome 84	Edge 84	Firefox 79	IE No	Opera No	Safari No	WebView Android 84	Chrome Android 84	Firefox Android No	Opera Android No	Safari iOS No	Samsung Internet Android No	nodejs 13.0.0 Disabled 13.0.0 Disabled Disabled From version 13.0.0: this feature is behind the `--harmony-weak-refs` runtime flag.
`WeakRef()` 构造函数	Chrome 84	Edge 84	Firefox 79	IE No	Opera No	Safari No	WebView Android 84	Chrome Android 84	Firefox Android No	Opera Android No	Safari iOS No	Samsung Internet Android No	nodejs 13.0.0 Disabled 13.0.0 Disabled Disabled From version 13.0.0: this feature is behind the `--harmony-weak-refs` runtime flag.
`deref`	Chrome 84	Edge 84	Firefox 79	IE No	Opera No	Safari No	WebView Android 84	Chrome Android 84	Firefox Android No	Opera Android No	Safari iOS No	Samsung Internet Android No	nodejs 13.0.0 Disabled 13.0.0 Disabled Disabled From version 13.0.0: this feature is behind the `--harmony-weak-refs` runtime flag.

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