Can't call $.data on object inside function

I'm playing around with the jQuery $.data function, and I'm running in to some trouble. If I do like this:

(function($){
    $.fn.testData = function() {
         var obj = $(this);
         obj.text($.data(obj,'test')); 
    }
})(jQuery);

var element = $("#test");
$.data(element,'test','hej');
element.testData();

this comes out as undefined. Why?

EDIT:

It works just fine if I use the elem.data(key) function, like this:

(function($){
    $.fn.testData = function() {
         var obj = $(this);
         obj.text(obj.data('test')); 
    }
})(jQuery);

var element = $("#test");
element.data('test','hej');
element.testData();

but I just saw an slideshow by Paul Irish, which claims that 开发者_StackOverflow中文版elem.data is 10x slower than $.data(elem): http://paulirish.com/2009/perf/

(function($){
    $.fn.testData = function() {
         var obj = $(this);
         obj.text($.data(obj[0],'test'));
    }

    var element = $("#test");
    $.data(element[0],'test','hej');
    element.testData();
})(jQuery);

Here is the jsFiddle: http://jsfiddle.net/TdJHq/

jQuery.data (doc jQuery.data) works on DOM elements, not on jQuery elements. So you must extract the real element under your jQuery selector.

obj.text($.data(obj[0],'test'));

This is what @alexl explained.

But there is also a .data() method, that works on a jQuery selector, so you could use:

// getter
obj.text($obj.data('test'));
// setter
obj.text($obj.data('test','toto'));

And you may have mixed both.

The problem is that you are creating different objects for attaching and retrieving the data. Whenever you call $(selector), you are creating a new jQuery object and this it will be different than the one you attached the data to.

Here is a simple example:

$.data($('#test'),'test','hej');
alert($.data($('#test'),'test'));

It will give you undefined.

That is why $.data expects a DOM element and not a jQuery object. No matter how you retrieve a certain DOM element, the reference is always the same.

So either pass the DOM element ($('#test')[0]) or better, use $('#test').data().