Promise polyfill with jQuery Deferreds

promise.js

function Promise(fn) {
  this.promise = $.Deferred(function(dfd) {
    fn(dfd.resolve, dfd.fail);
  }).promise();
}

Promise.prototype.then = function(done, fail) {
  this.promise.then(done, fail);
};

var promise = new Promise(function(resolve, fail) {
  setTimeout(function() {
    resolve();
  }, 3000);
});

promise.then(function() {
  console.log('done!');
});