code: function render(that, dom, options) {
    if (!dom) {
      if (that._root && that._root.children) {
        dom = that._root.children;
      } else {
        return '';
      }
    } else if (typeof dom === 'string') {
      dom = select(dom, that._root, options);
    }
  
    return serialize(dom, options);
  }
docs: /*
  Helper function