Super simplified Promise example.

Prrromise.js

const THEN = 'then';
const CATCH = 'catch';

class Prrromise {

  static get THEN() {
    return THEN;
  }

  static get CATCH() {
    return CATCH;
  }

  constructor(fn) {
    this.q = [];

    /** @TODO move out from constructor */
    this.then = this._pusher(Prrromise.THEN);
    this.catch = this._pusher(Prrromise.CATCH);

    this._onHandle = this._onHandle.bind(this);
    this._onReject = this._onReject.bind(this);

    fn(this._onHandle, this._onReject);
  }

  _onHandle(val) {
    try {

      for (let i = 0, l = this.q.length; i < l; i++) {
        const { type, fn } = this.q.shift();
        if (Prrromise.CATCH === type) continue;
        val = fn(val);
      }

    } catch(err) {
      this._onReject(err);
    }
  }

  _onReject(err) {
    let val;
    const i  = this.q.findIndex(({ type }) => type === Prrromise.CATCH);

    this.q = this.q.slice(i);
    const fn = this.q.shift();

    try {
      val = fn(err);
    } catch(err) {
      this._onReject(err);
    }

    this._onHandle(val);
  }

  _pusher(type) {
    return (fn) => {
      this.q.push({ fn, type });
      return this;
    };
  }

}

module.exports = Prrromise;