39 WeakSets (WeakSet) ^ES6 (advanced)

39.1 How are WeakSets different from Sets?

WeakSets are similar to Sets, with the following differences:

• They can hold objects without preventing those objects from being garbage-collected.

• They are black boxes: we only get any data out of a WeakSet if we have both the WeakSet and a value. The only methods that are supported are .add(), .delete(), .has(). See the section on WeakMaps as black boxes for an explanation of why WeakSets don’t allow iteration, looping, and clearing.

39.2 Use case for WeakSets: marking objects

Given that we can’t iterate over the elements of WeakSets, there are not that many use cases for them.

We can use WeakSets to mark objects – for example:

const isSaved = new WeakSet();
{
  const obj = {};
  isSaved.add(obj); // (A)
  assert.equal(
    isSaved.has(obj), // (B)
    true
  );
}
// (C)

• In line A, we mark obj as saved.
• In line B, we check if obj is saved.
• In line C, obj can be garbage-collected even though it is an element of the data structure isSaved.

In a way, we created a boolean property for obj, but stored it externally. If isSaved were a property, the previous code would look as follows:

{
  const obj = {};
  obj.isSaved = true;
  assert.equal(
    obj.isSaved,
    true
  );
}

39.2.1 Example: Marking objects as safe to use with a method

The following code demonstrates how a class can ensure that its methods are only applied to instances that were created by it (based on code by Domenic Denicola):

const instancesOfSafeClass = new WeakSet();

class SafeClass {
  constructor() {
    instancesOfSafeClass.add(this);
  }

  method() {
    if (!instancesOfSafeClass.has(this)) {
      throw new TypeError('Incompatible object!');
    }
  }
}

const safeInstance = new SafeClass();
safeInstance.method(); // works

assert.throws(
  () => {
    const obj = {};
    SafeClass.prototype.method.call(obj); // throws an exception
  },
  TypeError
);

39.3 WeakSet API

The constructor and the three methods of WeakSet work the same as their Set equivalents:

• new WeakSet<T>(values?: Iterable<T>) ^ES6
• .add(value: T): this ^ES6
• .delete(value: T): boolean ^ES6
• .has(value: T): boolean ^ES6