toc.js

 // Get ToC div
 toc = document.getElementById( "ToC" );

 //Add a header
 tocHeader = document.createElement( "h6" );
 tocHeader.innerText = "Contents";
 toc.appendChild( tocHeader );

 // Create a list for the ToC entries
 tocList = document.createElement( "ul" );

 // Get the h2 tags - ToC entries
 headers = document.getElementsByTagName( "h2" );

 // For each h3
 for ( i = 0; i < headers.length; i++ ) {

   // Create an id
   name = "h" + i;
   headers[ i ].id = name;

   // a list item for the entry
   tocListItem = document.createElement( "li" );

   // a link for the h3
   tocEntry = document.createElement( "a" );
   tocEntry.setAttribute( "href", "#" + name );
   tocEntry.innerText = headers[ i ].innerText;

   tocListItem.appendChild( tocEntry );
   tocList.appendChild( tocListItem );
 }
 toc.appendChild( tocList );