gistfile1.js

var promise = (function(){
	var undef;
	function Promise(){}
	Promise.prototype.constructor = Promise;
	
	return function(cb) {
		var publicAPI, queue = [], old_ret, promise_fulfilled = false;
		
		function fulfill(val) {
			var ret_val = val;
			if (typeof ret_val != "undefined") old_ret = ret_val;
			
			try {
				return val;
			}
			finally {
				for (var i=0,len=queue.length; i<len; i++) {
					if (typeof ret_val != "undefined") old_ret = ret_val;
					
					ret_val = queue[0].call(publicAPI,ret_val);
					
					if (typeof ret_val == "undefined") { ret_val = old_ret; }
					else if (ret_val && !(ret_val instanceof Promise) old_ret = ret_val;
										
					queue.shift();
					
					if (ret_val && ret_val instanceof Promise) {
						promise_fulfilled = false;
						ret_val.then(function(P){ promise_fulfilled = true; return (old_ret = fulfill(P.value)); });
						break;
					}
				}
			}
		}
				
		publicAPI = new Promise();
		
		publicAPI.then = function(cb){
			if (typeof cb == "function") queue[queue.length] = function(val){ return cb.call(publicAPI,{value:val}); }; // then() callback
			else queue[queue.length] = function(val){ return cb; }; // then() value
			if (promise_fulfilled) fulfill(old_ret);
			return publicAPI;
		};
		
		if (cb == null) { // empty promise
			promise_fulfilled = true;
		}
		else if (typeof cb == "function") { // promise callback
			cb.call(publicAPI,{fulfill:function(val){
				promise_fulfilled = true;
				fulfill.call(publicAPI,val);
			},value:undef});
		}
		else { // immediate promise value
			promise_fulfilled = true;
			return publicAPI.then(cb);
		}
		
		return publicAPI;
	};
})();

gistfile2.js

function xhrcall() {
   return promise(function(P){
      var xhr = new XMLHttpRequest();
      xhr.open("GET","some.tld/something");
      xhr.onreadystatechange = function(){
         if (xhr.readyState==4) P.fulfill(xhr.responseText);
      };
      xhr.send();
   });
}

xhrcall()
.then(function(P){
   alert(P.value);
});

gistfile3.js

function delay(secs) {
   return promise(function(P){
      setTimeout(P.fulfill,secs*1000);
   });
}

delay(5)
.then("Hello World!")
.then(function(P){
   alert(P.value);
});

gistfile4.js

// run xhrcall, then delay 10 secs, then alert the xhr response

xhrcall()
.then(function(P){
   return delay(10)
      .then(P.value)
   ;
})
.then(function(P){
   alert(P.value);
});

gistfile5.js

// run xhrcall and the delay in parallel, wait for both to fulfill,
// then alert the xhr response

var a = xhrcall();
var b = delay(10);

a.then(function(P){
   return b.then(P.value);
})
.then(function(P){
   alert(P.value);
});

gistfile6.js

// include: jQuery

function listenTo(whichElem) {
   return promise(function(P){
      $(whichElem).click(function(){P.fulfill(this);});
   });
}

listenTo("a")
.then(function(P){
   var elem = P.value;
   alert(elem.href);
});

gistfile7.js

function doSync(msg){
   return promise(msg);  // empty/immediate promise, for chaining
}

function doAsync(secs){
   return delay(secs);
}


doSync("starting...")
.then(function(P){
   alert(P.value);
   return doAsync(3);
})
.then(function(P){
   alert("...all done!");
});