题目描述：

上述表达式的输出结果为
['DIV']
['P', 'SPAN', 'P', 'SPAN']
['SPAN', 'SPAN']

直接上代码

let tree = document.querySelector(".a");
function traverseElRoot(elRoot) {
  const result = [];
  function traverse(element, level) {
    if (!result[level]) {
      result[level] = [];
    }
    result[level].push(element.tagName);
    Array.from(element.children).forEach((child) => {
      traverse(child, level + 1);
    });
  }
  traverse(elRoot, 0);
  return result;
}
console.log(traverseElRoot(tree));

输出