The documentation for the attr() method http://api.jquery.com/attr/ states:

>function(index, attr) A function returning the value to set. ''this is the current element.'' ...

The documentation for the addClass() method http://api.jquery.com/addClass/ doesn't give any such statement, but shows a code example using this as an apparent reference to the current element.

$("ul li:last").addClass(function() {
  return "item-" + $(this).index();
});

The docs for other methods such as append() and prepend() that allow a function passed to determine the value set, have no mention whatsoever of this as a reference to the current DOM element.

If it is jQuery's intention to provide this as a means of referencing the current DOM element in such functions, then please update the documentation for such methods in order to make it officially supported behavior.