const oldData = [
  {id:1,name:'boss',parentId:0},
  {id:2,name:'lily',parentId:1},
  {id:3,name:'jack',parentId:1},
  {id:4,name:'john',parentId:2},
  {id:5,name:'boss2',parentId:0},
]


function listToTree(oldArr){
  oldArr.forEach(element => {
    let parentId = element.parentId;
    if(parentId !== 0){
      oldArr.forEach(ele => {
        if(ele.id == parentId){ //当内层循环的ID== 外层循环的parendId时，（说明有children），需要往该内层id里建个children并push对应的数组；
          if(!ele.children){
            ele.children = [];
          }
          ele.children.push(element);
        }
      });
    }
  });
  console.log(oldArr) //此时的数组是在原基础上补充了children;
  oldArr = oldArr.filter(ele => ele.parentId === 0); //这一步是过滤，按树展开，将多余的数组剔除；
  console.log(oldArr)
  return oldArr;
}
listToTree(oldData);