Summary
Custom Animation function does not correctly handle the "unit" parameter if passed in an empty string
External Behavior
I first discovered this when I attempted to do something simple like this:
$("#someDiv").animate({color: "red"}, 1000);
Nothing happens when executing that.
(I forgot that jQuery doesn't support animation tweening between color values outside of plugins, but there is still a bug =)
Details
My first clue of what the problem was when my Firebug console spat out an error:
Expected color but found 'redpx'. Error in parsing value for property 'color'. Declaration dropped.
Digging into the code, in the uncompressed version of jQuery 1.2.2, we have:
2935 animate: function( prop, speed, easing, callback ) {
2936 var optall = jQuery.speed(speed, easing, callback);
2937
2938 return this[ optall.queue === false ? "each" : "queue" ](function(){
...
2963 jQuery.each( prop, function(name, val){
2964 var e = new jQuery.fx( self, opt, name );
2965
2966 if ( /toggle|show|hide/.test(val) )
2967 e[ val == "toggle" ? hidden ? "show" : "hide" : val ]( prop );
2968 else {
2969 var parts = val.toString().match(/^([+-]=)?([\d+-.]+)(.*)$/),
2970 start = e.cur(true) || 0;
2971
2972 if ( parts ) {
...
2988 } else
2989 e.custom( start, val, "" );
2990 }
2991 });
...
3122 jQuery.fx.prototype = {
...
3145 // Start an animation from one number to another
3146 custom: function(from, to, unit){
3147 this.startTime = (new Date()).getTime();
3148 this.start = from;
3149 this.end = to;
3150 this.unit = unit || this.unit || "px";
The problem is in lines 2989 and 3150.
On line 1989, an empty string is passed in as unit to custom.
On line 3150, the empty string is evaluated as falsey, this.unit is undefined, thus a unit of "px" is used.
Simply changing line 2989 to pass in a space instead of the empty string: e.custom( start, val, " ") fixed the issue and demonstrated that this indeed is the problem.
Note that although this bug was discovered when I tried to animate color in a way that wasn't meant to be used, this bug will manifest itself any time the code path executes line 2989.
