Iterable WeakSet in JavaScript

iterable_weak_set.js

// spec: https://github.com/tc39/proposal-weakrefs
// the spec contains an [iterable WeakMap implementation](https://github.com/tc39/proposal-weakrefs#iterable-weakmaps)
// NOTE: this WeakSet implementation is incomplete, only does what I needed
// In Firefox Nightly, visit about:config and enable javascript.options.experimental.weakrefs

class IterableWeakSet extends Set {
  add(el) {
    super.add(new WeakRef(el))
  }
  forEach(fn) {
    super.forEach(ref => {
      const value = ref.deref()
      if (value) fn(value)
    })
  }
  *[Symbol.iterator]() {
    for (const ref of super.values()) {
      const value = ref.deref()
      if (value) yield value
    }
  }
}

While I agree that it might be possible to implement a working weak set using the technique you suggest, this implementation has issues. It's a (quasi-) bag, not a set. There's no attempt to ensure that a single element is added to the set only once.

Indeed, this is neither a Set nor a WeakSet as it breaks all fundamental assumptions about them. To implement a class with a different behavior but a similar API, you should use composition, not inheritance.

If you wanted to support an API similar to WeakSet along with the iterable protocol, you'd need something similar to the IterableWeakMap found in the proposal mentioned in the gist's comment, but using added values as the internal WeakMap keys.