Promise.prototype.finally observability

promise-finally-observability.js

// Current spec
function finally(onFinally) {
  let C = SpeciesConstructor(this);
  return this.then(
    x => new C(r => r(onFinally())).then(() => x),
    e => new C(r => r(onFinally())).then(() => { throw e; })
  );
}

// Proposed, but rejected (potentially because of a miscommunication) spec
function finally(onFinally) {
  return builtInPromiseThen(
    this,
    x => builtinPromiseThen(BuiltinPromiseResolve(onFinally()), () => x),
    e => builtinPromiseThen(BuiltinPromiseResolve(onFinally()), () => { throw e; })
  );
}