Why is jQuery.data() modifying the HTML5 data-x attribute value?

Question

I have just encountered some very disturbing behavior in jQuery 1.6.2 that I hope someone can explain. Given the following markup...

<div id="test" data-test="    01">Test</div>

Can someone tell me why accessing the attribute through .data() causes it to be parsed to an int?

var t = $("#test").data("test");
alert(t);  // shows "1"

t = $("#test").attr("data-test");
alert(t);  // shows "    01"

Of course I have proof on jsFiddle of this behavior.

Answer 1

From the doc for data(key):

Every attempt is made to convert the string to a JavaScript value (this includes booleans, numbers, objects, arrays, and null) otherwise it is left as a string.

Since your example string CAN be converted to a number, it is.

Edit: As Joseph points out, this only applies when reading data straight from the element, like you're doing in this example. If you could set it first (i.e. data(key,value) before reading), then the behavior disappears. The getter, when reading from the actual DOM element, performs type coercion.