You should do
$("div.article h2").each(function(){
var title = $(this).text();
var slug = title.trim().toLowerCase().replace(" ", "_");
var list_item = "<li><a href='#"+slug+"'> " + title + "</a></li>";
$(this).prop('id', slug);
$(WHERE_YOU_WANT_TO_APPEND_IT).append(list_item);
}
Change WHERE_YOU_WANT_TO_APPEND_IT
by your selector