ECMAScript® 2024 Language Specification

Draft ECMA-262 / February 15, 2024

26.1 WeakRef Objects

A WeakRef is an object that is used to refer to a target object or symbol without preserving it from garbage collection. WeakRefs can be dereferenced to allow access to the target value, if the target hasn't been reclaimed by garbage collection.

26.1.1 The WeakRef Constructor

The WeakRef constructor:

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

26.1.1.1 WeakRef ( target )

This function performs the following steps when called:

  1. If NewTarget is undefined, throw a TypeError exception.
  2. If CanBeHeldWeakly(target) is false, throw a TypeError exception.
  3. Let weakRef be ? OrdinaryCreateFromConstructor(NewTarget, "%WeakRef.prototype%", « [[WeakRefTarget]] »).
  4. Perform AddToKeptObjects(target).
  5. Set weakRef.[[WeakRefTarget]] to target.
  6. Return weakRef.

26.1.2 Properties of the WeakRef Constructor

The WeakRef constructor:

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

26.1.2.1 WeakRef.prototype

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

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

26.1.3 Properties of the WeakRef Prototype Object

The WeakRef prototype object:

  • is %WeakRef.prototype%.
  • has a [[Prototype]] internal slot whose value is %Object.prototype%.
  • is an ordinary object.
  • does not have a [[WeakRefTarget]] internal slot.
Normative Optional

26.1.3.1 WeakRef.prototype.constructor

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

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

26.1.3.2 WeakRef.prototype.deref ( )

This method performs the following steps when called:

  1. Let weakRef be the this value.
  2. Perform ? RequireInternalSlot(weakRef, [[WeakRefTarget]]).
  3. Return WeakRefDeref(weakRef).
 Note

If the WeakRef returns a target value that is not undefined, then this target value should not be garbage collected until the current execution of ECMAScript code has completed. The AddToKeptObjects operation makes sure read consistency is maintained.

let target = { foo() {} };
let weakRef = new WeakRef(target);

// ... later ...

if (weakRef.deref()) {
  weakRef.deref().foo();
}

In the above example, if the first deref does not evaluate to undefined then the second deref cannot either.

26.1.3.3 WeakRef.prototype [ @@toStringTag ]

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

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

26.1.4 WeakRef Abstract Operations

26.1.4.1 WeakRefDeref ( weakRef )

The abstract operation WeakRefDeref takes argument weakRef (a WeakRef) and returns an ECMAScript language value. It performs the following steps when called:

  1. Let target be weakRef.[[WeakRefTarget]].
  2. If target is not empty, then
    1. Perform AddToKeptObjects(target).
    2. Return target.
  3. Return undefined.
 Note

This abstract operation is defined separately from WeakRef.prototype.deref strictly to make it possible to succinctly define liveness.

26.1.5 Properties of WeakRef Instances

WeakRef instances are ordinary objects that inherit properties from the WeakRef prototype. WeakRef instances also have a [[WeakRefTarget]] internal slot.