«Promise.js» — is supported as a native interface and $.Deferred.

Promise.js

/**
 * @author RubaXa <trash@rubaxa.org>
 * @license MIT
 */
(function () {
	"use strict";


	function noop() {
	}


	function _then(promise, method, callback) {
		return function () {
			var args = arguments, retVal;

			/* istanbul ignore else */
			if (typeof callback === 'function') {
				try {
					retVal = callback.apply(promise, args);
				} catch (err) {
					promise.reject(err);
					return;
				}

				if (retVal && typeof retVal.then === 'function') {
					if (retVal.done && retVal.fail) {
						retVal.done(promise.resolve).fail(promise.reject);
					}
					else {
						retVal.then(promise.resolve, promise.reject);
					}
					return;
				} else {
					args = [retVal];
					method = 'resolve';
				}
			}

			promise[method].apply(promise, args);
		};
	}


	/**
	 * «Обещания» поддерживают как [нативный](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise)
	 * интерфейс, так и [$.Deferred](http://api.jquery.com/category/deferred-object/).
	 *
	 * @class   Promise
	 * @param   {Function}  [executor]
	 * @returns {Promise}
	 */
	var Promise = function (executor) {
		var _completed = false;

		function _setState(state) {
			return function () {
				if (_completed) {
					return dfd;
				}

				_args = arguments;
				_completed = true;

				// Затираем методы
				dfd.done =
				dfd.fail =
				dfd.resolve =
				dfd.reject = function () {
					return dfd;
				};

				dfd[state ? 'done' : 'fail'] = function (fn) {
					/* istanbul ignore else */
					if (typeof fn === 'function') {
						fn.apply(dfd, _args);
					}
					return dfd;
				};

				var fn,
					fns = state ? _doneFn : _failFn,
					i = 0,
					n = fns.length
				;

				for (; i < n; i++) {
					fn = fns[i];
					/* istanbul ignore else */
					if (typeof fn === 'function') {
						fn.apply(dfd, _args);
					}
				}

				fns = _doneFn = _failFn = null;

				return dfd;
			};
		}

		var
			_args,
			_doneFn = [],
			_failFn = [],

			dfd = /** @lends Promise.prototype */ {

				/**
				 * Добавляет обработчик, который будет вызван, когда «обещание» будет «разрешено»
				 * @param  {Function}  fn  функция обработчик
				 * @returns {Promise}
				 */
				done: function (fn) {
					_doneFn.push(fn);
					return dfd;
				},

				/**
				 * Добавляет обработчик, который будет вызван, когда «обещание» будет «отменено»
				 * @param  {Function}  fn  функция обработчик
				 * @returns {Promise}
				 */
				fail: function (fn) {
					_failFn.push(fn);
					return dfd;
				},

				/**
				 * Добавляет сразу два обработчика
				 * @param   {Function}   [doneFn]   будет выполнено, когда «обещание» будет «разрешено»
				 * @param   {Function}   [failFn]   или когда «обещание» будет «отменено»
				 * @returns {Promise}
				 */
				then: function (doneFn, failFn) {
					var promise = Promise();

					dfd
						.done(_then(promise, 'resolve', doneFn))
						.fail(_then(promise, 'reject', failFn))
					;

					return promise;
				},

				notify: noop, // jQuery support
				progress: noop, // jQuery support
				promise: function () {
					// jQuery support
					return dfd;
				},


				/**
				 * Добавить обработчик «обещаний» в независимости от выполнения
				 * @param   {Function}   fn   функция обработчик
				 * @returns {Promise}
				 */
				always: function (fn) {
					return dfd.done(fn).fail(fn);
				},


				/**
				 * «Разрешить» «обещание»
				 * @method
				 * @param    {*}  result
				 * @returns  {Promise}
				 */
				resolve:  _setState(true),


				/**
				 * «Отменить» «обещание»
				 * @method
				 * @param   {*}  result
				 * @returns {Promise}
				 */
				reject: _setState(false)
			}
		;


		/**
		 * @name  Promise#catch
		 * @alias fail
		 * @method
		 */
		dfd['catch'] = function (fn) {
			return dfd.then(null, fn);
		};


		// Работеам как native Promises
		/* istanbul ignore else */
		if (typeof executor === 'function') {
			try {
				executor(dfd.resolve, dfd.reject);
			} catch (err) {
				dfd.reject(err);
			}
		}

		return dfd;
	};


	/**
	 * Дождаться «разрешения» всех обещаний
	 * @static
	 * @memberOf Promise
	 * @param    {Array} iterable  массив значений/обещаний
	 * @returns  {Promise}
	 */
	Promise.all = function (iterable) {
		var dfd = Promise(),
			d,
			i = 0,
			n = iterable.length,
			remain = n,
			values = [],
			_fn,
			_doneFn = function (i, val) {
				(i >= 0) && (values[i] = val);

				/* istanbul ignore else */
				if (--remain <= 0) {
					dfd.resolve(values);
				}
			},
			_failFn = function (err) {
				dfd.reject([err]);
			}
		;

		if (remain === 0) {
			_doneFn();
		}
		else {
			for (; i < n; i++) {
				d = iterable[i];

				if (d && typeof d.then === 'function') {
					_fn = _doneFn.bind(null, i); // todo: тест
					if (d.done && d.fail) {
						d.done(_fn).fail(_failFn);
					} else {
						d.then(_fn, _failFn);
					}
				}
				else {
					_doneFn(i, d);
				}
			}
		}

		return dfd;
	};


	/**
	 * Дождаться «разрешения» всех обещаний и вернуть результат последнего
	 * @static
	 * @memberOf Promise
	 * @param    {Array}   iterable   массив значений/обещаний
	 * @returns  {Promise}
	 */
	Promise.race = function (iterable) {
		return Promise.all(iterable).then(function (values) {
			return values.pop();
		});
	};


	/**
	 * Привести значение к «Обещанию»
	 * @static
	 * @memberOf Promise
	 * @param    {*}   value    переменная или объект имеющий метод then
	 * @returns  {Promise}
	 */
	Promise.cast = function (value) {
		var promise = Promise().resolve(value);
		return value && typeof value.then === 'function'
			? promise.then(function () { return value; })
			: promise
		;
	};


	/**
	 * Вернуть «разрешенное» обещание
	 * @static
	 * @memberOf Promise
	 * @param    {*}   value    переменная
	 * @returns  {Promise}
	 */
	Promise.resolve = function (value) {
		return Promise().resolve(value);
	};


	/**
	 * Вернуть «отклоненное» обещание
	 * @static
	 * @memberOf Promise
	 * @param    {*}   value    переменная
	 * @returns  {Promise}
	 */
	Promise.reject = function (value) {
		return Promise().reject(value);
	};


	/**
	 * Дождаться «разрешения» всех обещаний
	 * @param   {Object}  map «Ключь» => «Обещание»
	 * @returns {Promise}
	 */
	Promise.map = function (map) {
		var array = [], key, idx = 0, results = {};

		for (key in map) {
			array.push(map[key]);
		}

		return Promise.all(array).then(function (values) {
			/* jshint -W088 */
			for (key in map) {
				results[key] = values[idx++];
			}

			return results;
		});
	};



	// Версия модуля
	Promise.version = "0.2.1";


	// exports
	if (typeof define === "function" && (define.amd || /* istanbul ignore next */ define.ajs)) {
		define('Promise', [], function () {
			return Promise;
		});
	} else if (typeof module != "undefined" && module.exports) {
		module.exports = Promise;
	}
	else {
		window.Deferred = Promise;

		/* istanbul ignore else */
		if (!window.Promise) {
			window.Promise = Promise;
		}
	}
})();

Promise.test.js

define(['Promise', 'jquery'], function (MyPromise, $) {
	/* jshint asi: true */
	module('Promise');


	requireTest('core', function (Promise) {
		var promise = new Promise, name;

		for (name in { resolve: 1, reject: 1, all: 1, race: 1, cast: 1 }) {
			equal(typeof Promise[name], 'function', name);
		}

		for (name in { done: 1, fail: 1, then: 1, 'catch': 1, resolve: 1, reject: 1, always: 1 }) {
			equal(typeof promise[name], 'function', name);
		}
	});



	function _checkStatus(actual, expected) {
		return function (args) {
			equal(actual, expected);
			deepEqual(args, ["foo", "bar"]);
		};
	}


	// Test for 'resolve' and 'reject'
	for (var key in { resolve: 1, reject: 1 }) {
		/*jshint loopfunc: true */
		(function (method) {
			asyncTest(method, function () {
				expect(4);

				var dfd = MyPromise();

				dfd
					.done(_checkStatus(method, 'resolve'))
					.fail(_checkStatus(method, 'reject'))
					.then(_checkStatus(method, 'resolve'), _checkStatus(method, 'reject'))
				;

				dfd[method](["foo", "bar"]);

				dfd.always(function () {
					setTimeout(start, 0);
				});
			});


			asyncTest(method + ' x 2', function () {
				expect(8);

				var dfd = MyPromise();

				dfd
					.done(_checkStatus(method, 'resolve'))
					.fail(_checkStatus(method, 'reject'))
					.then(_checkStatus(method, 'resolve'), _checkStatus(method, 'reject'))
				;

				dfd[method](["foo", "bar"]);
				dfd[method](["bar", "foo"]);

				dfd
					.done(_checkStatus(method, 'resolve'))
					.fail(_checkStatus(method, 'reject'))
					.then(_checkStatus(method, 'resolve'), _checkStatus(method, 'reject'))
				;

				dfd.always(function () {
					setTimeout(start, 0);
				});
			});


			asyncTest(method + '..' + method, function () {
				expect(4);

				var dfd = MyPromise();

				dfd[method](["foo", "bar"]);

				dfd
					.done(_checkStatus(method, 'resolve'))
					.fail(_checkStatus(method, 'reject'))
					.then(_checkStatus(method, 'resolve'), _checkStatus(method, 'reject'))
				;

				dfd[method](["bar", "foo"]);

				dfd.always(function () {
					setTimeout(start, 0);
				});
			});
		})(key);
	}


	asyncTest('then', function () {
		var log = [];

		MyPromise()
			.done(function (a) {
				log.push(a)
			})
			.resolve(1)
			.then(function (a) {
				return MyPromise().reject(a + 2)
			})
			.done(function () {
				log.push('fail')
			})
			.fail(function (a) {
				log.push(a)
			})
			.then(null, function (a) {
				return MyPromise().resolve(a + 3);
			})
			.done(function (a) {
				log.push(a)
			})
			.fail(function () {
				log.push('fail')
			})
			.then(function (a) {
				return a * 2;
			})
			.done(function (a) {
				log.push(a)
			})
			.fail(function () {
				log.push('fail')
			})
		;

		equal(log.join('->'), '1->3->6->12');


		var fail = false;
		MyPromise(function (resolve, reject) {
			setTimeout(function () {
				reject('reject');
			}, 50);
		})
			.then(null, function (val) {
				return MyPromise(function (resolve) {
					setTimeout(function () {
						resolve(val + ' resolve');
					}, 50);
				});
			})
			.always(function (res) {
				ok(!fail);
				equal(res, 'reject resolve');

				MyPromise.reject()
					.then(function () {
						return 'fail';
					}, function () {
						return {
							then: function (done, fail) {
								fail('ok');
							}
						};
					})
					.always(function (res) {
						equal(res, 'ok');

						new MyPromise(function () {
							throw '1';
						})['catch'](function (x) {
							throw x + '2'
						})['catch'](function (x) {
							return x;
						}).then(function (x) {
							equal(x, '12');
							start();
						});
					})
				;
			})
		;
	});


	asyncTest('then + jquery', function () {
		$.Deferred()
			.reject('foo')
			.then(null, function (val) {
				return MyPromise.resolve(val + ' bar');
			})
			.always(function (res) {
				equal(res, 'foo bar');
				start();
			})
		;
	});


	asyncTest('then + fail', function () {
		var n = [], m = [];

		Promise.reject()
			.then(function () { n.push('1Y'); }, function () { n.push('1N'); })
			.then(function () { n.push('2Y'); }, function () { n.push('2N'); })
			.then(function () {
			})
		;

		setTimeout(function () {
			Promise.resolve()
				.then(function () { n.push('3Y'); }, function () { n.push('3N'); })
				.then(function () { n.push('4Y'); }, function () { n.push('4N'); })
			;
		}, 3);

		MyPromise.reject()
			.then(function () { m.push('1Y'); }, function () { m.push('1N'); })
			.then(function () { m.push('2Y'); }, function () { m.push('2N'); })
		;

		setTimeout(function () {
			MyPromise.resolve()
				.then(function () { m.push('3Y'); }, function () { m.push('3N'); })
				.then(function () { m.push('4Y'); }, function () { m.push('4N'); })
			;
		}, 3);

		setTimeout(function () {
			deepEqual(m, n);
			start();
		}, 10);
	});


	asyncTest('all: done', function () {
		expect(4);

		var foo = MyPromise(),
			bar = MyPromise(),
			baz = MyPromise(),
			qux = { // like Promise
				dfd: MyPromise(),
				resolve: function (val){ this.dfd.resolve(val); },
				then: function (done){ this.dfd.then(done); }
			},
			empty
		;

		setTimeout(foo.resolve.bind(null, 1), 100);
		bar.resolve(2);

		setTimeout(baz.resolve.bind(null, 3), 150);
		qux.resolve(4);

		MyPromise.all([]).always(function (){
			empty = true;
		});


		MyPromise.all([foo, bar, '|', baz, qux, 'YES!', null, false, 100500])
			.done(function () { ok(true, "done"); })
			.then(function (values) { ok(true, "then"); return values })
				.fail(function () { ok(false, "fail"); })
				.then(function (values) { return values }, function () { ok(false, "then(null, fail)"); })
					.always(function (values) {
						ok(empty, 'empty');
						equal(values.join('->'), '1->2->|->3->4->YES!->->false->100500');

						setTimeout(start, 0);
					})
		;
	});


	asyncTest('all: fail', function () {
		expect(2);

		var foo = MyPromise(),
			bar = MyPromise(),
			baz = MyPromise(),
			qux = MyPromise()
		;

		setTimeout(foo.resolve, 100);
		bar.resolve();

		setTimeout(baz.reject, 150);
		qux.resolve();

		MyPromise.all([foo, bar, baz, qux])
			.done(function () { ok(false, "done"); })
			.then(function () { ok(false, "then"); })
				.fail(function () { ok(true, "fail"); })
				.then(null, function () { ok(true, "then(null, fail)"); })
					.always(function () {
						setTimeout(start, 0);
					})
		;
	});


	asyncTest('like Native', function () {
		MyPromise(function (resolve) {
			resolve('foo');
		}).always(function (val) {
				equal(val, 'foo');
				start();
			});
	});


	test('static: resolve/reject', function () {
		expect(2);
		MyPromise.resolve('foo').done(function (val) {
			equal(val, 'foo')
		});
		MyPromise.reject('bar').fail(function (val) {
			equal(val, 'bar')
		});
	});


	test('race', function () {
		expect(1);

		MyPromise.race([
			MyPromise.resolve(1),
			MyPromise.resolve(2)
		]).then(function (value) {
			return value * 3;
		}).always(function (value){
			return equal(value, 6);
		});
	});


	test('cast', function () {
		expect(3);

		MyPromise.cast(3).then(function (result) {
			equal(result, 3);
		});

		MyPromise.cast($.Deferred().resolve('resolve')).then(function (result) {
			equal(result, 'resolve');
		});

		MyPromise.cast($.Deferred().reject('reject')).catch(function (result) {
			equal(result, 'reject');
		});
	});


	asyncTest('stress', function () {
		var log = {};

		function testMe(method) {
			var dfd = MyPromise();
			var name = method == 'resolve' ? 'done' : 'fail';

			dfd[name](function () {
				log[name] = true;
			});
			dfd[name](dfd[method]);

			dfd[method]();

			return dfd;
		}


		MyPromise.all([testMe('resolve'), testMe('reject')]).always(function () {
			ok(log.done);
			ok(log.fail);
			start();
		});
	});


	asyncTest('resolve + reject', function () {
		new MyPromise(function (resolve, reject) {
			resolve(true);

			setTimeout(function () {
				try {
					reject(false);
					ok(true);
				} catch (err) {
					equal(err+'', null);
				}
				start();
			});
		});
	});


	asyncTest('catch', function () {
		new Promise(function () {
			throw "foo";
		})['catch'](function (foo) {
			return new MyPromise(function () {
				throw "bar";
			})['catch'](function (bar) {
				return [foo, bar];
			});
		}).then(function (values) {
			deepEqual(values, ['foo', 'bar']);
			start();
		}, function (err) {
			equal(err+'', null, 'fail');
			start();
		});
	});


	promiseTest('map', function () {
		return MyPromise.map({
			foo: MyPromise.resolve(1),
			bar: MyPromise.resolve(2)
		}).then(function (data) {
			deepEqual(data, { foo: 1, bar: 2 });
		});
	});


	/**
	test('bench', function () {
		var NativePromise = window.Promise, i;

		var ts = new Date;
		for (i = 0; i < 1e4; i++) {
			MyPromise().resolve().then(function () {});
		}
		ts = (new Date) - ts;

		var $ts = new Date;
		for (i = 0; i < 1e4; i++) {
			$.Deferred().resolve().then(function () {});
		}
		$ts = (new Date) - $ts;

		var nts = new Date;
		if (NativePromise) {
			for (i = 0; i < 1e4; i++) {
				NativePromise.resolve().then(function () {});
			}
		}
		nts = (new Date) - nts;

		NativePromise && ok(ts / nts < 2, 'Native, RubaXa win: ' + (nts / ts));
		ok($ts / ts > 5, 'jQuery, RubaXa win: ' + ($ts / ts));

		console.log('Promise: ' + ts + 'ms');
		console.log('Native.Promise: ' + nts + 'ms');
		console.log('jQuery.Deferred: ' + $ts + 'ms');
	});
	/**/
});

Thanks for this nice, compact Deferred solution.

I encountered an issue with Deferred.when: when resolved, the results of the resolved Deferred objects are not passed.

@josdejong Updated solution.
Now a full-fledged polyfill + is compatible with jQuery.

@RubaXa Great tiny library, thanks. It also got a favourable comparison here. Do you have plans to move it to a repo? It would be nice to have it at hand as bower dependency .

By any chance, do you have a TypeScript definitions for this gist? Thanks!