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.