Fade на чистом JS (типа FadeTo в jQuery)

function fade( o, opts, dontStartNow ) {
	var opacityVal, opacityStart, opacityEnd, opacityStep, t;
 
	function th() {
		opacityVal += opacityStep;
		if ( ( opacityStep > 0 && opacityVal >= opacityEnd ) || ( opacityStep < 0 && opacityVal <= opacityEnd ) ) {
			opacityVal = opacityEnd;
			clearInterval( t );
			t = null;
		}
 
		o.style.opacity = opacityVal;
 
		if ( !t && opts.hasOwnProperty( "handler" ) ) opts.handler( true, fs, o );
	}
 
	function init() {
		opacityStep = opts.hasOwnProperty( "opacityStep" ) ? Math.abs( opts.opacityStep ) : 0.1;
 
		if ( !opts.hasOwnProperty( "delay" ) ) opts.delay = 30;
 
		if ( !opts.hasOwnProperty( "opacityStart" ) ) {
			opacityStart = parseFloat( o.style.opacity );
			if ( isNaN( opacityStart ) ) opacityStart = 1;
		}
		else {
			opacityStart = opts.opacityStart;
			o.style.opacity = opacityStart;
		}
 
		opacityVal = opacityStart;
 
		if ( !opts.hasOwnProperty( "opacityEnd" ) ) {
			opacityEnd = parseFloat( o.style.opacity );
			if ( isNaN( opacityEnd ) ) opacityEnd = 1;
		}
		else opacityEnd = opts.opacityEnd;
 
 
		if ( opacityStart > opacityEnd ) opacityStep = -opacityStep;
 
		if ( opacityStart != opacityEnd ) t = setInterval( th, opts.delay );
	}
 
	var fs = {
		get: function() {
			return {
				opts:		opts,
				opacityVal:	opacityVal,
				opacityEnd:	opacityEnd,
				opacityStep:opacityStep
			};
		},
 
		stop: function( end, dontNotify ) {
			if ( !t ) return;
 
			clearInterval( t );
			t = null;
			if ( end ) o.style.opacity = opacityEnd;
 
			if ( dontNotify !== true && opts.hasOwnProperty( "handler" ) ) opts.handler( true, fs, o );
		},
 
		start: function( restart, newOpts, dontNotify ) {
			if ( t ) return;
 
			if ( newOpts ) opts = newOpts;
 
			if ( restart ) {
				init();
				if ( dontNotify !== true && opts.hasOwnProperty( "handler" ) ) opts.handler( false, fs, o );
			}
			else t = setInterval( th, opts.delay );
		}
	};
 
	if ( dontStartNow !== true ) fs.start( true );
	return fs;
}

var f = fade( o, opts );