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){ //當內層循環的ID== 外層循環的parendId時，（說明有children），需要往該內層id里建個children并push對應的數組；          if(!ele.children){            ele.children = [];          }          ele.children.push(element);        }      });    }  });  console.log(oldArr) //此時的數組是在原基礎上補充了children;  oldArr = oldArr.filter(ele => ele.parentId === 0); //這一步是過濾，按樹展開，將多余的數組剔除；  console.log(oldArr)  return oldArr;}listToTree(oldData);