JavaScript数组对象：如何合并属性值相同的元素？

<code class="javascript">const arr = [
  { title: '标题1', url: 'url', groupId: -1 },
  { title: '标题2', url: 'url', groupId: 123 },
  { title: '标题3', url: 'url', groupId: 123 },
  { title: '标题4', url: 'url', groupId: -1 },
  { title: '标题5', url: 'url', groupId: 456 },
  { title: '标题6', url: 'url', groupId: 456 },
  { title: '标题7', url: 'url', groupId: 789 },
];

const resArr = arr.reduce((previous, current) => {
  const last = previous.length ? previous[previous.length - 1] : null;

  if (current.groupId === -1) {
    previous.push({ title: current.title, url: current.url }); // groupId为-1的元素单独处理
  } else if (last && last.groupId === current.groupId) {
    last.group.push({ title: current.title, url: current.url }); // 合并到已有组
  } else {
    previous.push({ groupId: current.groupId, group: [{ title: current.title, url: current.url }] }); // 创建新组
  }

  return previous;
}, []);

console.log(resArr);</code>