将对象的平面数组转换为嵌套对象的数组

const flatArray = [
  { id: 1, name: 'Root', parentId: null },
  { id: 2, name: 'Child1', parentId: 1 },
  { id: 3, name: 'Child2', parentId: 1 },
  { id: 4, name: 'GrandChild', parentId: 2 }
];

function buildNestedStructure(flatArray) {
  const map = {};
  const roots = [];

  flatArray.forEach(item => {
    map[item.id] = { ...item, children: [] };
  });

  flatArray.forEach(item => {
    if (item.parentId !== null) {
      map[item.parentId].children.push(map[item.id]);
    } else {
      roots.push(map[item.id]);
    }
  });

  return roots;
}

const nestedArray = buildNestedStructure(flatArray);
console.log(JSON.stringify(nestedArray, null, 2));

[
  {
    "id": 1,
    "name": "Root",
    "parentId": null,
    "children": [
      {
        "id": 2,
        "name": "Child1",
        "parentId": 1,
        "children": [
          {
            "id": 4,
            "name": "GrandChild",
            "parentId": 2,
            "children": []
          }
        ]
      },
      {
        "id": 3,
        "name": "Child2",
        "parentId": 1,
        "children": []
      }
    ]
  }
]