tiny Promise.js

// (c) copyright unscriptable.com / John Hann | LegendLee
// License MIT
// For more robust promises, see https://github.com/briancavalier/when.js.
// last edited by LegendLee(legendlee1314@gmail.com)

function Promise() {
  this._thens = [];
}

Promise.prototype = {
  then: function(onFulfilled, onRejected) {
    this._thens.push({fulfill: onFulfilled, reject: onRejected});
    return this;
  },
  fulfill: function(value) {
    this._done('fulfill', value);
  },
  reject: function(error) {
    this._done('reject', error);
  },
  _done: function(which, arg) {
    // Cover and sync func `then()`.
    this.then = which === 'fulfill' ?
      function(fulfill, reject) {fulfill && fulfill(arg); return this;} :
      function(fulfill, reject) {reject && reject(arg); return this;};
    // Disallow multiple calls.
    this.fulfill = this.reject =
      function() {throw new Error('Promise already completed.');}
    // Complete all async `then()`s.
    var then, i = 0;
    while (then = this._thens[i++]) {
      then[which] && then[which](arg);
    }
    delete this._thens;
  }
};