beforeunload events in Internet Explorer 9

Chrome is my weapon of choice for JavaScript development. This means spending a little bit of time with IE9 when development's almost over, learning to play with its little ways. We were taking beforeunload to display a dialog whenever the user risks losing data by navigating away — but only then. IE9 (oddly) displays null in a dialog when null is returned from beforeunload. Returning undefined instead means harmony amongst browsers. Another little trick to add to the playbook...