TimeLord.js: hack javascript's sense of time

timelord.js

window.TimeLord = (function(){

	/*
		# Time accounted for
			Date.now()
			window.performance.now()
			window.setInterval(...)
			window.setTimeout(...)
			window.requestAnimationFrame(...)
		# Not accounted for
			(new Date()).getTime()
	*/

	var fakeTime_ms = 0;
	var activateStamp = 0;

	var intervalIdCounter = 1;
	var intervals = [];

	function stepTime(dt_ms){
		fakeTime_ms += dt_ms;

		var i = intervals.length;
		while(--i >= 0){
			var iObj = intervals[i];
			var dt = fakeTime_ms - iObj.start_ms;

			if(dt >= iObj.ms){
				//fire
				iObj.fn.apply(window, iObj.fnArgs);

				if(iObj.loop){
					iObj.start_ms += iObj.ms;
				}else{
					//remove
					intervals.splice(i, 1);
				}
			}
		}
	}

	function setTime(ms){
		if(ms < fakeTime_ms){
			console.warn('TimeLord: without a tardis or flux capacitor you\'re probably going to have errors if you try to go back in time');
		}
		var dt_ms = ms - fakeTime_ms;
		stepFakeTime(dt_ms);//call stepTime so our intervals get fired
	}

	function getTime(){
		return fakeTime_ms;
	}

	function addInterval(fn, ms, loop, fnArgs){
		var id = intervalIdCounter++;
		intervals.push({
			id: id,
			fn: fn,
			ms: ms,
			loop: loop,
			fnArgs: fnArgs,
			start_ms: fakeTime_ms,
		});
		return id;
	}

	function removeInterval(id){
		var i = intervals.length;
		while(--i >= 0){
			if(intervals[i].id === id){
				intervals.splice(i, 1);
				break;
			}
		}
	}

	function activate(){
		//override javascript time functions
		activateStamp = Date.now();

		//absolute time
		Date._now = Date.now;
		Date.now = function(){
			return activateStamp + fakeTime_ms;
		}

		window.performance._now = window.performance.now;
		window.performance.now = function(){
			return fakeTime_ms;
		}

		//relative time
		window._setTimeout = window.setTimeout;
		window.setTimeout = function(fn, ms){
			var fnArgs = Array.prototype.slice.call(arguments, [2]);
			return addInterval(fn, ms, false, fnArgs);
		}

		window._setInterval = window.setInterval;
		window.setInterval = function(fn, ms){
			var fnArgs = Array.prototype.slice.call(arguments, [2]);
			return addInterval(fn, ms, true, fnArgs);
		}

		window._clearInterval = window.clearInterval;
		window.clearInterval = function(id){
			removeInterval(id);
		}

		window._clearTimeout = window.clearTimeout;
		window.clearTimeout = function(id){
			removeInterval(id);
		}

		window._requestAnimationFrame = window.requestAnimationFrame;
		window.requestAnimationFrame = function(cb){
			var newCb = function(realTime_ms){
				cb(fakeTime_ms);
			}
			return window._requestAnimationFrame(newCb);
		}
	}

	return {
		activate: activate,
		stepTime: stepTime,
		setTime: setTime,
		getTime: getTime
	};
})();

Original functions accessed by prefixing _, eg: Date._now()

Example usage: run time at 1/2 speed

var multiplier = 0.5;

var lastRealTime_ms = window.performance._now();

function updateRealTime(realTime_ms){
    var dt = realTime_ms - lastRealTime_ms;
    TimeLord.stepTime(dt * multiplier);
    lastRealTime_ms = realTime_ms;

    window._requestAnimationFrame(updateRealTime);
}

updateRealTime(lastRealTime_ms);

Thanks