ECMAScript® 2021 Language Specification

Draft ECMA-262 / March 3, 2021

26.2 FinalizationRegistry Objects

A FinalizationRegistry is an object that manages registration and unregistration of cleanup operations that are performed when target objects are garbage collected.

26.2.1 The FinalizationRegistry Constructor

The FinalizationRegistry constructor:

  • is %FinalizationRegistry%.
  • is the initial value of the "FinalizationRegistry" property of the global object.
  • creates and initializes a new FinalizationRegistry object when called as a constructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • is designed to be subclassable. It may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified FinalizationRegistry behaviour must include a super call to the FinalizationRegistry constructor to create and initialize the subclass instance with the internal state necessary to support the FinalizationRegistry.prototype built-in methods.

26.2.1.1 FinalizationRegistry ( cleanupCallback )

When the FinalizationRegistry function is called with argument cleanupCallback, the following steps are taken:

  1. If NewTarget is undefined, throw a TypeError exception.
  2. If IsCallable(cleanupCallback) is false, throw a TypeError exception.
  3. Let finalizationRegistry be ? OrdinaryCreateFromConstructor(NewTarget, "%FinalizationRegistry.prototype%", « [[Realm]], [[CleanupCallback]], [[Cells]] »).
  4. Let fn be the active function object.
  5. Set finalizationRegistry.[[Realm]] to fn.[[Realm]].
  6. Set finalizationRegistry.[[CleanupCallback]] to cleanupCallback.
  7. Set finalizationRegistry.[[Cells]] to a new empty List.
  8. Return finalizationRegistry.

26.2.2 Properties of the FinalizationRegistry Constructor

The FinalizationRegistry constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has the following properties:

26.2.2.1 FinalizationRegistry.prototype

The initial value of FinalizationRegistry.prototype is the FinalizationRegistry prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

26.2.3 Properties of the FinalizationRegistry Prototype Object

The FinalizationRegistry prototype object:

  • is %FinalizationRegistry.prototype%.
  • has a [[Prototype]] internal slot whose value is %Object.prototype%.
  • is an ordinary object.
  • does not have [[Cells]] and [[CleanupCallback]] internal slots.

26.2.3.1 FinalizationRegistry.prototype.constructor

The initial value of FinalizationRegistry.prototype.constructor is %FinalizationRegistry%.

26.2.3.2 FinalizationRegistry.prototype.register ( target, heldValue [ , unregisterToken ] )

The following steps are taken:

  1. Let finalizationRegistry be the this value.
  2. Perform ? RequireInternalSlot(finalizationRegistry, [[Cells]]).
  3. If Type(target) is not Object, throw a TypeError exception.
  4. If SameValue(target, heldValue) is true, throw a TypeError exception.
  5. If Type(unregisterToken) is not Object, then
    1. If unregisterToken is not undefined, throw a TypeError exception.
    2. Set unregisterToken to empty.
  6. Let cell be the Record { [[WeakRefTarget]]: target, [[HeldValue]]: heldValue, [[UnregisterToken]]: unregisterToken }.
  7. Append cell to finalizationRegistry.[[Cells]].
  8. Return undefined.
 Note

Based on the algorithms and definitions in this specification, cell.[[HeldValue]] is live when cell is in finalizationRegistry.[[Cells]]; however, this does not necessarily mean that cell.[[UnregisterToken]] or cell.[[Target]] are live. For example, registering an object with itself as its unregister token would not keep the object alive forever.

26.2.3.3 FinalizationRegistry.prototype.unregister ( unregisterToken )

The following steps are taken:

  1. Let finalizationRegistry be the this value.
  2. Perform ? RequireInternalSlot(finalizationRegistry, [[Cells]]).
  3. If Type(unregisterToken) is not Object, throw a TypeError exception.
  4. Let removed be false.
  5. For each Record { [[WeakRefTarget]], [[HeldValue]], [[UnregisterToken]] } cell of finalizationRegistry.[[Cells]], do
    1. If cell.[[UnregisterToken]] is not empty and SameValue(cell.[[UnregisterToken]], unregisterToken) is true, then
      1. Remove cell from finalizationRegistry.[[Cells]].
      2. Set removed to true.
  6. Return removed.

26.2.3.4 FinalizationRegistry.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "FinalizationRegistry".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

26.2.4 Properties of FinalizationRegistry Instances

FinalizationRegistry instances are ordinary objects that inherit properties from the FinalizationRegistry prototype. FinalizationRegistry instances also have [[Cells]] and [[CleanupCallback]] internal slots.