code: function find_child(parent, name, attr) {
      let children = parent.childNodes;
      for (let i = 0; i < children.length; i++) {
          if (children[i].tagName == name)
              return (attr != undefined) ? children[i].attributes[attr].value : children[i];
      }
      return;
  }
docs: 以下是公共方法