code: function _groupBy(collection, getter) {
          var result = {};
          var prop;
  
          forEach( collection, function( elm ) {
            prop = getter(elm);
  
            if(!result[prop]) {
              result[prop] = [];
            }
            result[prop].push(elm);
          });
          return result;
        }
docs: groupBy function
  @param collection
  @param getter
  @returns {{}}