Преобразование в иерархическую структуру

conversion.js

const source = [
    { id: 1 },
    { id: 3, parentId: 1 },
    { id: 4, parentId: 3 },
    { id: 5, parentId: 3 },
    { id: 6, parentId: 1 },
    { id: 7, parentId: 6 },
    { id: 8, parentId: 7 },
    { id: 9, parentId: null }
];

const res = source
    .map(item => Object.assign( {}, item ))
    .reduce((acc, curr, index, arr) => {
      if (curr.parentId) {
        let parent = arr.find(item => item.id === curr.parentId);
        (parent.children = parent.children || []).push(curr);
      } else {
        acc.push(curr);
      }
        return acc;
    }, []);

console.log(res);
/**
[
  {
    "id": 1,
    "children": [
      {
        "id": 3,
        "parentId": 1,
        "children": [...]
      },
      {
        "id": 6,
        "parentId": 1,
        "children": [...]
      }
    ]
  },
  {
    "id": 9,
    "parentId": null
  }
]
*/