Schedule a function to run as often as possible, featuring a difference_in_time from the last time it ran, an adjustable clock_with_velocity for bullet time, and a frame counter with which you can build your own FPS meter.

animate.js

(function (win, undefined){
// Schedule a function to run as often as possible,
// featuring a difference_in_time from the last time it ran,
// an adjustable clock_with_velocity for bullet time,
// and a frame counter with which you can build your own FPS meter.
	
var gettime = function (){
	return (new Date).getTime()
},
animate = (function(){		// animate(fn, starting_velocity)
				//   fn(
				//	dt,
				//	frame = Timeline.pos,
				//	Timeline = {time(), last_t, pos, vel}
				//   )

	function Timeline(vel, pos){
		this.last_t = gettime(),
		this.vel = vel || 1,
		this.pos = pos || 0
	
		this.time = function (num){
			// quantum time measurement, disturbs the time
			if (num) this.pos = num
			var t = this.last_t
			this.last_t = gettime(),
			this.pos = this.pos + this.vel * (this.last_t - t)
			return this.last_t
		}
	}
	
	return function(fn, vel){
		// teh interface
		var run = true, last_t, clock = new Timeline(vel),
		Loop = function (t){
			// needs requestAnimationFrame polyfill
			if (run) window.requestAnimationFrame(Loop)
			else return
		
			last_t = clock.last_t || clock.time()
			t = clock.time()  // starts at 'frame 1' because
 
			var dt = clock.vel * Math.abs(t - last_t)
			run = fn(dt, clock, clock.pos)
			dt = null
		}
		Loop()
	}
})()
 
win.animate = animate
})(window)
 

test.js

// USAGE example
animate(function(dt, t, frame){
	// this is playing with the clock speed, ignore
	if (t.set_vel === void 0)
		t.set_vel = t.vel = t.vel * 2
	if (frame > 200 && t.unset_vel === void 0)
		t.unset_vel = t.vel = t.vel / 2
		// why do I even import undefined if I never end up using it?
	console.log(t.last_t, dt,
		frame, t.vel)  // frame == t.pos
	if (frame < 300)
		return true  // stop animation at frame 300
}, .5)  // starting at half speed