Deferred.js

(function() {

  function Deferred() {
    if (window.Promise) {
      var that = this;
      this.promise = new Promise(function(resolve, reject) {
        that._resolve = resolve;
        that._reject = reject;
      });
    }
  }

  Deferred.prototype.resolve = function(value) {
    if (this._resolve)
      this._resolve(value);
  };

  Deferred.prototype.reject = function(reason) {
    if (this._reject)
      this._reject(reason);
  };

  window.Deferred = Deferred;
  
})();

usage.js

function async() {
  var def = new Deferred();
  def.resolve();
  return def.promise;
}