window.onload = stripeDivs; 

   function stripeDivs() { 

      var tDivs = document.getElementById('listings').getElementsByTagName('div'); 

      for (var i=0, tDiv; tDiv=tDivs[i]; i++) { 

         if (i%2) { 
            tDiv.className += (tDiv.className) ? ' listingAlt' : 'listingAlt'; 
         } 

      } 

   }