WeakSet

A WeakSet is a collection of garbage-collectable values, including objects and non-registered symbols. A value in the WeakSet may only occur once. It is unique in the WeakSet's collection.

Description

Values of WeakSets must be garbage-collectable. Most can be arbitrarily created and don't have a lifetime, so they cannot be stored. Objects and non-registered symbols can be stored because they are garbage-collectable.

The main differences to the Set object are:

WeakSets are collections of objects and symbols only. They cannot contain arbitrary values of any type, as Sets can.
The WeakSet is weak, meaning references to objects in a WeakSet are held weakly. If no other references to a value stored in the WeakSet exist, those values can be garbage collected.

Note: This also means that there is no list of current values stored in the collection. WeakSets are not enumerable.

Use case: Detecting circular references

Functions that call themselves recursively need a way of guarding against circular data structures by tracking which objects have already been processed.

WeakSets are ideal for this purpose:

// Execute a callback on everything stored inside an object
function execRecursively(fn, subject, _refs = new WeakSet()) {
  // Avoid infinite recursion
  if (_refs.has(subject)) {
    return;
  }

  fn(subject);
  if (typeof subject === "object" && subject) {
    _refs.add(subject);
    for (const key in subject) {
      execRecursively(fn, subject[key], _refs);
    }
    _refs.delete(subject);
  }
}

const foo = {
  foo: "Foo",
  bar: {
    bar: "Bar",
  },
};

foo.bar.baz = foo; // Circular reference!
execRecursively((obj) => console.log(obj), foo);

Here, a WeakSet is created on the first run, and passed along with every subsequent function call (using the internal _refs parameter).

The number of objects or their traversal order is immaterial, so a WeakSet is more suitable (and performant) than a Set for tracking object references, especially if a very large number of objects is involved.

Constructor

WeakSet()
- : Creates a new WeakSet object.

Instance properties

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

WeakSet.prototype.constructor
- : The constructor function that created the instance object. For WeakSet instances, the initial value is the WeakSet constructor.
WeakSet.prototype[@@toStringTag]
- : The initial value of the @@toStringTag property is the string "WeakSet". This property is used in Object.prototype.toString.

Instance methods

WeakSet.prototype.add
- : Appends value to the WeakSet object.
WeakSet.prototype.delete
- : Removes value from the WeakSet. WeakSet.prototype.has(value) will return false afterwards.
WeakSet.prototype.has
- : Returns a boolean asserting whether value is present in the WeakSet object or not.

Examples

Using the WeakSet object

const ws = new WeakSet();
const foo = {};
const bar = {};

ws.add(foo);
ws.add(bar);

ws.has(foo); // true
ws.has(bar); // true

ws.delete(foo); // removes foo from the set
ws.has(foo); // false, foo has been removed
ws.has(bar); // true, bar is retained

Note that foo !== bar. While they are similar objects, they are not the same object. And so they are both added to the set.