JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/3.10.1/lodash.min.js"></script>

JavaScript

var data = getData();

var group = _.groupBy(data, 'parent');

console.log(group);

function getData() {
    return {
        "1": {
            "id": 1,
            "name": "foo",
            "parent": null,
            "root": 1,
            "children": [2, 4, 6],
            "items":[
                { "id": "1", "name": "item1" },
                { "id": "2", "name": "item2" },
                { "id": "3", "name": "item3" }
            ]
        },
        "2": {
            "id": 2,
            "name": "bar",
            "parent": 1,
            "root": 1,
            "children": null,
            "items":[
                { "id": "4", "name": "item4" }
            ]
        },
        "3": {
            "id": 3,
            "name": "bazz",
            "parent": null,
            "root": 3,
            "children": [5, 7],
            "items":[
                { "id": "5", "name": "item5" },
                { "id": "6", "name": "item6" }
            ]
        },
        "4": {
            "id": 4,
            "name": "zap",
            "parent": 1,
            "root": 1,
            "children": [9],
            "items":[
                { "id": "7", "name": "item7" },
                { "id": "8", "name": "item8" },
                { "id": "9", "name": "item9" },
                { "id": "10", "name": "item10" }
            ]
        },
        "5": {
            "id": 5,
            "name": "lel",
            "parent": 3,
            "root": 1,
            "children": null,
            "items":[]
        },
        "6": {
            "id": 6,
            "name": "pop",
            "parent": 1,
            "root": 1,
            "children": null,
            "items":[
                { "id": "11", "name": "item11" },
                { "id": "12", "name": "item12" }
            ]
        },
        "7": {
            "id": 7,
            "name": "jiz",
            "parent": 3,
            "root": 3,
            "children": null,
            "items":[]
       ...