Promise.js

function Promise(){
	
	var self = this;
	var thenCallback = null;
	var failCallback = null;
	var alwaysCallback = null;

	self.then = function(callback){
		thenCallback = callback;
		return self;
	};

	self.error = function(callback){
		failCallback = callback;
		return self;
	};

	self.always = function(callback){
		alwaysCallback = callback;
		return self;
	};

	self.complete = function(args){
		runCallback(alwaysCallback, args);
		runCallback(thenCallback, args);
	};

	self.fail = function(args){
		runCallback(alwaysCallback, args);
		runCallback(failCallback, args);
	}

	function runCallback(callbackToRun, args){
		if (callbackToRun && typeof callbackToRun === 'function'){
			callbackToRun(args);
		}
	}
}

var p = new Promise();

p.then(function(args){
	console.log('foo has happened ' + args);
}).error(function(args){
	console.log('error has happened');
}).always(function(args){
	console.log('something has happened, good or bad');
});

p.complete(12);
p.fail();


function myFunction(){
	
	var p = new Promise();
	setTimeout(p.complete, 1000)
	return p;
}

var promise = myFunction()
promise.then(function(){
	console.log('done');
});