When .attr(attrName) is called on a selection that does not have an attrName attribute, undefined is returned.

When .attr(attrName, undefined) is called on a selection, the attrName attribute is not removed. Instead .attr(attrName, null) must be called to remove the attrName attribute from the selection of elements.

For consistency, I see two options being available:

Option 1

<div data-example="example"></div>
<script>
  $('div').attr('data-example', undefined);
</script>

should result in:

<div></div>

Option 2

<div></div>
<script>
  console.log($('div').attr('data-example'));
</script>

should result in:

null

http://jsfiddle.net/gtSh5/