在此页

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

A FinalizationRegistry object lets you request a callback when an object is garbage-collected.

描述

FinalizationRegistry provides a way to request that a cleanup callback get called at some point when an object registered with the registry has been reclaimed (garbage-collected). (Cleanup callbacks are sometimes called finalizers .)

注意： Cleanup callbacks should not be used for essential program logic. See Notes on cleanup callbacks 了解细节。

You create the registry passing in the callback:

const registry = new FinalizationRegistry(heldValue => {
  // ....
});
					

Then you register any objects you want a cleanup callback for by calling the `register` method, passing in the object and a *held value* for it:

registry.register(theObject, "some value");
					

The registry does not keep a strong reference to the object, as that would defeat the purpose (if the registry held it strongly, the object would never be reclaimed).

若 theObject is reclaimed, your cleanup callback may be called at some point with the held value you provided for it ( "some value" in the above). The held value can be any value you like: a primitive or an object, even undefined . If the held value is an object, the registry keeps a strong reference to it (so it can pass it to your cleanup callback later).

If you might want to unregister an object later, you pass a third value, which is the unregistration token you'll use later when calling the registry's unregister function to unregister the object. The registry only keeps a weak reference to the unregister token.

It's common to use the object itself as the unregister token, which is just fine:

registry.register(theObject, "some value", theObject);
// ...some time later, if you don't care about `theObject` anymore...
registry.unregister(theObject);
					

It doesn't have to be the same object, though; it can be a different one:

registry.register(theObject, "some value", tokenObject);
// ...some time later, if you don't care about `theObject` anymore...
registry.unregister(tokenObject);
					

构造函数

FinalizationRegistry()

创建新的 FinalizationRegistry 对象。

实例方法

FinalizationRegistry.prototype.register()

Registers an object with the registry in order to get a cleanup callback when/if the object is garbage-collected.

FinalizationRegistry.prototype.unregister()

Unregisters an object from the registry.

Avoid where possible

Correct use of FinalizationRegistry 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 that FinalizationRegistry is part of included in its explainer document :

Garbage collectors are complicated. If an application or library depends on GC cleaning up a FinalizationRegistry 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 cleanup callbacks

Some notes on cleanup callbacks:

• Developers shouldn't rely on cleanup callbacks for essential program logic. Cleanup callbacks may be useful for reducing memory usage across the course of a program, but are unlikely to be useful otherwise.
• A conforming JavaScript implementation, even one that does garbage collection, is not required to call cleanup callbacks. When and whether it does so is entirely down to the implementation of the JavaScript engine. When a registered object is reclaimed, any cleanup callbacks for it may be called then, or some time later, or not at all.
• It's likely that major implementations will call cleanup callbacks at some point during execution, but those calls may be substantially after the related object was reclaimed.
• There are also situations where even implementations that normally call cleanup callbacks are unlikely to call them:
  • When the JavaScript program shuts down entirely (for instance, closing a tab in a browser).
  • When the FinalizationRegistry instance itself is no longer reachable by JavaScript code.

范例

Creating a new registry

You create the registry passing in the callback:

const registry = new FinalizationRegistry(heldValue => {
  // ....
});
					

Registering objects for cleanup

Then you register any objects you want a cleanup callback for by calling the `register` method, passing in the object and a *held value* for it:

registry.register(theObject, "some value");
					

规范

规范
WeakRefs The definition of 'FinalizationRegistry' 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
FinalizationRegistry	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.
FinalizationRegistry() 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.
register	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.
unregister	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.

图例

完整支持

完整支持

不支持

不支持

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

• WeakRef
• WeakSet
• WeakMap

元数据

• 最后修改： Aug 12, 2020

相关话题

1. 标准内置对象
2. FinalizationRegistry
3. 方法
   1. FinalizationRegistry.prototype.register()
   2. FinalizationRegistry.prototype.unregister()
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()