Simple JavaScript Animation Engine

simple_animation_engine.js

function now(){
	return ( new Date ).getTime();
}

function animate( time, fn, fps ){
  var start = now(),
	intrval = fps ? 1000/fps : 20,
	id = setInterval(function(){
  			var diff = now() - start, p = diff/time;
  			
  			if( p > 1 ) p = 1;
	  			fn( p );
  			if( p == 1 ) clearInterval( id );
  			
		}, intrval);
}