Dummy promise

promise.js

var Promise = (function () {
    "use strict";

    function worker(queue, val) {
        return queue.reduce(function (res, obj, i) {
            try {
                return obj.fn(res);
            } catch (ex) {
                return obj.err(ex);
            }
        }, val);
    }
    
    function Promise() {
        this.res = null;
        this.queue = [];
    }

    Promise.prototype.when = function (fn) {
        this.res = fn();
        return this;
    };

    Promise.prototype.then = function (fn, err) {
        this.queue.push({fn: fn, err: err});
        return this;
    };

    Promise.prototype.done = function (fn, err) {
        this.queue.push({fn: fn, err: err});
        this.res = worker(this.queue, this.res);

        return this.res;
    };

    return Promise;
}());

var promise = new Promise();

var res = promise.when(function () {
      console.log("when " + 1);
      return 1 + 1;
  }).then(function (res) {
      console.log("then " + res);
      return Math.pow(res, res);
  }, function (err) {
      console.log("err " + err);
      return 1;
  }).done(function (res) {
      console.log("done " + res);
      return res * 4;
  }, function (err) {
      console.log("err " + err);
      return 15;
  });