How to remove all classes with jQuery?

Just use

jQuery('selector').removeClass();

to remove all the classes.

Use attr():

selector$.attr("class", "");

From the documentation:

If a class name is included as a parameter, then only that class will be removed from the set of matched elements. If no class names are specified in the parameter, all classes will be removed.

So simply using:

$('#whatever').removeClass();

will remove all classes from #whatever.