code: function(filter) {
        return u(array.filter.call(this, function(el, index) {
          return /^f/.test(typeof filter) ? filter(index, el) : u(el).is(filter);
        }));
      }
docs: filter method
  filter elements by selector or filter function
  @param  {string|function} filter - selector or filter function
  @return {object}                   matching elements