In my opinion, having read parts of jQuery's source code on various occasions… You should only read it for entertainment value, it makes for fascinating brain teasers. Here's one snippet I had the opportunity to read recently:

  contains = hasCompare || rnative.test( docElem.contains ) ?
    function( a, b ) {
      var adown = a.nodeType === 9 ? a.documentElement : a,
        bup = b && b.parentNode;
      return a === bup || !!( bup && bup.nodeType === 1 && (
        adown.contains ?
          adown.contains( bup ) :
          a.compareDocumentPosition && a.compareDocumentPosition( bup ) & 16
      ));
    } :
    function( a, b ) {
      if ( b ) {
        while ( (b = b.parentNode) ) {
          if ( b === a ) {
            return true;
          }
        }
      }
      return false;
    };