function Promise(handler, parent) {
  var me = this;

  if (!(this instanceof Promise)) {
    throw new SyntaxError('Constructor must be called with the new operator');
  }

  if (typeof handler !== 'function') {
    throw new SyntaxError('Function parameter handler(resolve, reject) missing');
  }

  var _onSuccess = [];
  var _onFail = [];

  // status
  /**
   * @readonly
   */
  this.resolved = false;
  /**
   * @readonly
   */
  this.rejected = false;
  /**
   * @readonly
   */
  this.pending = true;
  /**
   * @readonly
   */
  this[Symbol.toStringTag] = 'Promise';

  /**
   * Process onSuccess and onFail callbacks: add them to the queue.
   * Once the promise is resolved, the function _promise is replace.
   * @param {Function} onSuccess
   * @param {Function} onFail
   * @private
   */
  var _process = function (onSuccess, onFail) {
    _onSuccess.push(onSuccess);
    _onFail.push(onFail);
  };

  /**
   * Add an onSuccess callback and optionally an onFail callback to the Promise
   * @template TT
   * @template [TE=never]
   * @param {(r: T) => TT | PromiseLike<TT>} onSuccess
   * @param {(r: E) => TE | PromiseLike<TE>} [onFail]
   * @returns {Promise<TT | TE, any>} promise
   */
  this.then = function (onSuccess, onFail) {
    return new Promise(function (resolve, reject) {
      var s = onSuccess ? _then(onSuccess, resolve, reject) : resolve;
      var f = onFail    ? _then(onFail,    resolve, reject) : reject;

      _process(s, f);
    }, me);
  };

  /**
   * Resolve the promise
   * @param {*} result
   * @type {Function}
   */
  var _resolve = function (result) {
    // update status
    me.resolved = true;
    me.rejected = false;
    me.pending = false;

    _onSuccess.forEach(function (fn) {
      fn(result);
    });

    _process = function (onSuccess, onFail) {
      onSuccess(result);
    };

    _resolve = _reject = function () { };

    return me;
  };

  /**
   * Reject the promise
   * @param {Error} error
   * @type {Function}
   */
  var _reject = function (error) {
    // update status
    me.resolved = false;
    me.rejected = true;
    me.pending = false;

    _onFail.forEach(function (fn) {
      fn(error);
    });

    _process = function (onSuccess, onFail) {
      onFail(error);
    };

    _resolve = _reject = function () { }

    return me;
  };

  /**
   * Cancel the promise. This will reject the promise with a CancellationError
   * @returns {this} self
   */
  this.cancel = function () {
    if (parent) {
      parent.cancel();
    }
    else {
      _reject(new CancellationError());
    }

    return me;
  };

  /**
   * Set a timeout for the promise. If the promise is not resolved within
   * the time, the promise will be cancelled and a TimeoutError is thrown.
   * If the promise is resolved in time, the timeout is removed.
   * @param {number} delay     Delay in milliseconds
   * @returns {this} self
   */
  this.timeout = function (delay) {
    if (parent) {
      parent.timeout(delay);
    }
    else {
      var timer = setTimeout(function () {
        _reject(new TimeoutError('Promise timed out after ' + delay + ' ms'));
      }, delay);

      me.always(function () {
        clearTimeout(timer);
      });
    }

    return me;
  };

  // attach handler passing the resolve and reject functions
  handler(function (result) {
    _resolve(result);
  }, function (error) {
    _reject(error);
  });
}
Promise

Inspired by https://gist.github.com/RubaXa/8501359 from RubaXa <trash@rubaxa.org>
@template T
@template [E=Error]
@param {Function} handler   Called as handler(resolve: Function, reject: Function)
@param {Promise} [parent]   Parent promise for propagation of cancel and timeout