Promise wrapper with state getters (isPending, isFulfilled, isRejected)

promise-with-state.js

/*
 * Create a new promise the same way as you would with the Promise constructor:
 *
 *   let p = new PromiseWithState((res, rej) => {
 *     // ...
 *   });
 *
 *   p.isPending(); // true
 *   p.isFulfilled(); // false
 *   p.isRejected(); // false
 *
 * Also works with Promise static methods:
 *
 *   let p = PromiseWithState.resolve('foo');
 *
 *   p.isPending(); // false
 *   p.isFulfilled(); // true
 */

const PromiseWithState = Object.setPrototypeOf(function(fn) {
  let states = {
    isPending: true,
    isFulfilled: false,
    isRejected: false
  };

  let p = new Promise(fn).then(
    val => {
      states.isFulfilled = true;
      return val;
    },
    err => {
      states.isRejected = true;
      throw err;
    }
  ).finally(() => {
    states.isPending = false;
  });

  Object.keys(states).forEach(state => {
    Object.defineProperty(p, state, {
      enumerable: false,
      get() {
        return () => states[state];
      }
    });
  });

  return p;
}, Promise);