Proxies can be used for data-binding, similar to Object.observe.

I've also yet to find a case where I need WeakMaps instead of an Object or Map. All the benefits seem to be a bit too theoretical, but I likely haven't dug into it hard enough.

Assume an immutable object may undergo an expensive transform that returns an object, which is then stored in a WeakMap keyed to the immutable. When there are no new references to that object, the garbage collector will clean it up, but otherwise the expensive transform may be avoided with a lookup on the WeakMap.

It provides a pretty nice way to decouple your caching from the rest of the codebase.

    [Symbol.iterator]() {
      if (this.left) {
        yield* this.left[Symbol.iterator]();
      }
      yield this.value;
      if (this.right) {
        yield* this.rightSymbol.iterator]();
      }
    }
    ...
    for (const value of myTree) { ... }