WeakRef

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

Description

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.

Because non-registered symbols are also garbage collectable, they can also be used as the target of a WeakRef object. However, the use case of this is limited.

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 included by the authors in the proposal that introduced WeakRef:

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

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 WeakRefs 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, the 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.

Constructor

WeakRef()
- : Creates a new WeakRef object.

Instance properties

These properties are defined on WeakRef.prototype and shared by all WeakRef instances.

WeakRef.prototype.constructor
- : The constructor function that created the instance object. For WeakRef instances, the initial value is the WeakRef constructor.
  
  Note: This property is marked as "normative optional" in the specification, which means a conforming implementation may not expose the constructor property. This prevents arbitrary code from obtaining the WeakRef constructor and being able to observe garbage collection. However, all major engines do expose it by default.
WeakRef.prototype[@@toStringTag]
- : The initial value of the @@toStringTag property is the string "WeakRef". This property is used in Object.prototype.toString.

Instance methods

WeakRef.prototype.deref
- : Returns the WeakRef object's target object, or undefined if the target object has been reclaimed.

Examples

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"));
setTimeout(() => {
  document.getElementById("counter").remove();
}, 5000);