Table of Contents generation in JavaScript using jQuery

contents.js

function contents() { 
   var first = $('h2:first'); 
   if (!first) return; 

   var p = '<p><strong>Contents</strong>'; 
   var ol = '<ol id="contents"></ol>'; 
   first.before('<div class="contents">' + p + ol + '</div>'); 

   var outer = $('#contents'); 
   $('h2, h3, h4').map(function (i) { 
      var header = $(this); 
      header.attr('id', 'tmp.' + (i + 1)); 

      var link = '<a href="#tmp.' + (i + 1) + '">' + header.text() + '</a>'; 
      var item = '<li class="' + this.tagName + '">' + link; 

      if (typeof previous == 'undefined') {
         outer.append(item) 
      } else if (this.tagName > previous) {
         $('li:last', outer).append('<ul>' + item + '</ul>') 
      } else if (this.tagName < previous) {
         $('ul:last', outer).parent().after(item) 
      } else { $('li:last', outer).after(item) }
      previous = this.tagName; 
   }); 
}

$(document).ready(contents);