Flat data to tree

Source data ：

let arr = [
    {id: 1, name: ' department 1', pid: 0},
    {id: 2, name: ' department 2', pid: 1},
    {id: 3, name: ' department 3', pid: 1},
    {id: 4, name: ' department 4', pid: 3},
    {id: 5, name: ' department 5', pid: 4},
];

Need output ：

[
    {
        "id": 1,
        "name": " department 1",
        "pid": 0,
        "children": [
            {
                "id": 2,
                "name": " department 2",
                "pid": 1,
                "children": [
                    {...}
                ]
            },
            {
                "id": 3,
                "name": " department 3",
                "pid": 1,
                "children": [
                    {...}
                ]
            }
        ]
    }
]

The easiest way , Cycle after sorting （filter It can also cycle ）

function ToTree(arr){
    return arr
            .sort((a, b) => b.pid - a.pid)
            .filter(item=>{
                item.children = arr.filter(subitem=> subitem.pid === item.id);
                return item.pid ===0;
            })
}

In terms of performance , Data can be converted to Map

function arrayToTree(arr) {
  const result = [];   //  Store results 
  const mapData = {};  // 
  for (let item of arr) {
    const id = item.id;
    const pid = item.pid;

    if (!mapData[id]) {
      mapData[id] = {
        children: [],
      }
    }

    mapData[id] = {
      ...item,
      children: mapData[id]['children']
    }

    const treeItem =  mapData[id];

    if (pid === 0) {
      result.push(treeItem);
    } else {
      if (!mapData[pid]) {
        mapData[pid] = {
          children: [],
        }
      }
      mapData[pid].children.push(treeItem)
    }

  }
  return result;
}