in_groups_of.js

window.inGroupsOf = function(array, number, fillWith) {
   fillWith = fillWith || null;
   var index = -number, slices = [];

   if (number < 1) return array;

   while ((index += number) < array.length) {
       var s = array.slice(index, index + number);
       while(s.length < number)
           s.push(fillWith);
       slices.push(s);
   }
   return slices;
};