Typescript, generic, функция. Типизация возвращаемого значения

const data = [
  {
    _id: "6258460841beb0b276d1312e",
    name: 'root1',
    status: 'root',
    description: 'root1-desc',
  },
  {
    _id: "6258463e41beb0b276d13135",
    parentId: "6258460841beb0b276d1312e",
    name: 'branch1.1',
    status: 'branch',
    description: 'branch1.1-desc',
  },
  {
    _id: "6259c7af6ab55bfcc4060524",
    name: 'root2',
    status: 'root',
    description: 'root2-desc',
  },
  {
    _id: "6259c7bc6ab55bfcc4060528",
    parentId: "6258463e41beb0b276d13135",
    properties: [],
    name: 'leaf1.1',
    status: 'leaf',
    description: 'leaf-desc',
  },
  {
    _id: "625aed38e43c85ac38c05636",
    parentId: "6259c7af6ab55bfcc4060524",
    name: 'branch2.1',
    status: 'branch',
    description: 'branch2.1-desc',
  }
]


interface ITreeDataEl {
    _id: string
    name: string
    parentId?: string
}

interface ITree {  // ????
    _id: string
    name: string
    children: Array<ITree>
}


function createTrees<T extends ITreeDataEl>(data: T[] , parentId?: string) {
    let d: T[] = []; 
    const trees: ITree[] = [];  // ????
    if(!parentId) {
        d = data.filter((item) => {
            return item.parentId === undefined;
        });
    }
    else {
        d = data.filter((item) => {
            return item.parentId?.toString() === parentId.toString(); 
        });
    }
    for(let i=0; i<d.length; i+=1) {
        const tree = {...d[i], children: createTrees(data, d[i]._id)};
        delete tree.parentId;
        trees.push(tree);
    }
    return trees;
}


const trees = createTrees(data);

[
  {
    _id: "6258460841beb0b276d1312e",
    name: 'root1',
    status: 'root',
    description: 'root1-desc',
    children: [     
      {
        _id: "6258463e41beb0b276d13135",
        parentId: "6258460841beb0b276d1312e",
        name: 'branch1.1',
        status: 'branch',
        description: 'branch1.1-desc',
        children: [
          {
            _id: "6259c7bc6ab55bfcc4060528",
            parentId: "6258463e41beb0b276d13135",
            properties: [],
            name: 'leaf1.1',
            status: 'leaf',
            description: 'leaf-desc',
          },
        ]
      },
    ]
  },
  {
    _id: "6259c7af6ab55bfcc4060524",
    name: 'root2',
    status: 'root',
    description: 'root2-desc',
    children: [ 
      {
        _id: "625aed38e43c85ac38c05636",
        parentId: "6259c7af6ab55bfcc4060524",
        name: 'branch2.1',
        status: 'branch',
        description: 'branch2.1-desc',
        children: [],
      },
    ]
  },
]