UNB/ CS/ David Bremner/ teaching/ cs2613/ books/ mdn/ Reference/ Global Objects/ FinalizationRegistry

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

Description

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

Note: Cleanup callbacks should not be used for essential program logic. See Notes on cleanup callbacks for details.

You create the registry passing in the callback:

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

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

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

The registry does not keep a strong reference to the value, as that would defeat the purpose (if the registry held it strongly, the value would never be reclaimed). In JavaScript, objects and non-registered symbols are garbage collectable, so they can be registered in a FinalizationRegistry object as the target or the token.

If target 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 a registered target value 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 value. The registry only keeps a weak reference to the unregister token.

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

registry.register(target, "some value", target);
// …

// some time later, if you don't care about `target` anymore, unregister it
registry.unregister(target);

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

registry.register(target, "some value", token);
// …

// some time later
registry.unregister(token);

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 included by the authors in the proposal that introduced FinalizationRegistry:

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:

Notes on cleanup callbacks

Constructor

Instance properties

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

Instance methods

Examples

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");

Callbacks never called synchronously

No matter how much pressure you put on the garbage collector, the cleanup callback will never be called synchronously. The object may be reclaimed synchronously, but the callback will always be called sometime after the current job finishes:

let counter = 0;
const registry = new FinalizationRegistry(() => {
  console.log(`Array gets garbage collected at ${counter}`);
});

registry.register(["foo"]);

(function allocateMemory() {
  // Allocate 50000 functions — a lot of memory!
  Array.from({ length: 50000 }, () => () => {});
  if (counter > 5000) return;
  counter++;
  allocateMemory();
})();

console.log("Main job ends");
// Logs:
// Main job ends
// Array gets garbage collected at 5001

However, if you allow a little break between each allocation, the callback may be called sooner:

let arrayCollected = false;
let counter = 0;
const registry = new FinalizationRegistry(() => {
  console.log(`Array gets garbage collected at ${counter}`);
  arrayCollected = true;
});

registry.register(["foo"]);

(function allocateMemory() {
  // Allocate 50000 functions — a lot of memory!
  Array.from({ length: 50000 }, () => () => {});
  if (counter > 5000 || arrayCollected) return;
  counter++;
  // Use setTimeout to make each allocateMemory a different job
  setTimeout(allocateMemory);
})();

console.log("Main job ends");

There's no guarantee that the callback will be called sooner or if it will be called at all, but there's a possibility that the logged message has a counter value smaller than 5000.

Specifications

Browser compatibility

See also