fejw-polyfill-promise-skeleton.js

const state = {
  PENDING: 'pending',
  FULFILLED: 'fulfilled',
  REJECTED: 'rejected'
};

class MyPromise {
  constructor(executor) {
    // initial state
    this._state = state.PENDING;
    this._value = undefined;
    this._error = undefined;

    // a list to save promise chains before promise is settled
    this._thenQueue = [];

    const resolve = this._onFulfilled.bind(this);
    const reject = this._onRejected.bind(this);
    executor(resolve, reject);
  }

  /** start: pseudo internal methods */
  _onFulfilled(value) {}

  _onRejected(error) {}
  /** end: pseudo internal methods */

  /** start: exposed methods */
  then(fulfillFn, rejectFn) {}

  catch(rejectFn) {}
  /** end: exposed methods */
}