const nodes = [
    { id: 3, name: '节点C', pid: 1 },
    { id: 6, name: '节点F', pid: 3 },
    { id: 0, name: 'root', pid: null },
    { id: 1, name: '节点A', pid: 0 },
    { id: 8, name: '节点H', pid: 4 },
    { id: 4, name: '节点D', pid: 1 },
    { id: 2, name: '节点B', pid: 0 },
    { id: 5, name: '节点E', pid: 2 },
    { id: 7, name: '节点G', pid: 2 },
    { id: 9, name: '节点I', pid: 5 }
];

function convert(data) {
    if (!Array.isArray(data)) {
        new Error('data must be an array');
        return;
    }

    let result;
    let map = {};
    data.forEach(item => {
        //id作为key
        map[item.id] = item;
    });
    data.forEach(item => {
        // item.pid 为0时 返回underfined
        let parent = map[item.pid];
        if (parent) {
            //如果用children，就直接push,没有就初始化数组
            (parent.children || (parent.children = [])).push(item);
        } else {
            // 这里push的item是pid为0的数据
            result = item;
        }
    });
    return result;
}

console.log(convert(nodes));

{
    id: 0,
    name: 'root',
    pid: null,
    children: [
      { id: 1, name: '节点A', pid: 0, children: [Array] },
      { id: 2, name: '节点B', pid: 0, children: [Array] }
    ]
  }