WeakRef : Object

constructor

A WeakRef allows holding on to another object, while still allowing that object to be garbage collected if there are no other strong references to it. Note, JavaScript makes no guarantees on if or when an object will be garbage collected. See also FinalizationRegistry.

ECMAScript 2021 Spec

Constructors

new WeakRef(target : Object) : WeakRef

Creates a new WeakRef that tracks target.

Spec

Instance Methods

deref() : Object

Returns the target of this if it has not been garbage collected yet, otherwise returns undefined. If deref() returns a valid object, that object will not be garbage collected until control returns to the event loop, such as when returning from a callback or awaiting a Promise.

Example:

const sleep = (ms) => new Promise(r => setTimeout(r, ms));

let foo = {};
const weakRef = new WeakRef(foo);
foo = undefined; // Clear strong reference

const startTime = Date.now();
console.log('Allocating a lot of objects to force garbage collection');
while (weakRef.deref()) {
  for (let i = 0; i < 100; i++) {
    const x = new Array(100);
  }
  await sleep(10);
}
console.log(`foo was reclaimed after ${((Date.now() - startTime) / 1000).toFixed(1)}s`);

Run

Results:

Spec

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