MooTools Array inGroupsOf method. Returns array of arrays grouped by <size> elements.

inGroupsOf.js

Array.implement({
  inGroupsOf: function(size) {
    var slices = [];
    var current = [];
    this.each(function(e) {
      if (current.length < size) current.push(e);
      else {
        slices.push(current);
        current = [e];
      }
    });
    slices.push(current);
    return slices;
  }
});