親子関係のあるオブジェクトを生成する

HTML

<ul>
 <li><a href="https://teratail.com/questions/84105">JavaScript - javaScriptで親子関係のあるのオブジェクトを作る(84105)｜teratail</a></li>
</ul>

JavaScript

'use strict';
(function () {
  function getElementById (elementList, id) {
    // id = String(id);

    for (var i = 0, len = elementList.length, element; i < len; ++i) {
      element = elementList[i];

      if (element.id === id) {
        return element;
      }
    }

    return null;
  }

  function appendChild (parentObject, childObject) {
    Array.isArray(parentObject.children) ? parentObject.children.push(childObject) : parentObject.children = [childObject];
  }

  var before = [
    {
      "id": 1,
      "parent": 0
    },
    {
      "id": 2,
      "parent": 0
    },
    {
      "id": 3,
      "parent": 1
    },
    {
      "id": 4,
      "parent": 2
    },
    {
      "id": 5,
      "parent": 3
    }
  ];

  var rootObject = {"id": 0};

  before.forEach(function (object, i) {
    if (!Array.isArray(object.children)) {
      object.children = [];
    }

    appendChild(object.parent !== 0 ? getElementById(before, object.parent) : rootObject, object);
  });

  // 期待する整形後のコード
  var after = [
    {
      "id": 1,
      "parent": 0,
      "children": [
        {
          "id": 3,
          "parent": 1,
          "children": [
            {
              "id": 5,
              "parent": 3,
              "children": []  // 追加した
            }
          ]
        }
      ]
    },
    {
      "id": 2,
      "parent": 0,
      "children": [
        {
          "id": 4,
          "parent": 2,
          "children": []  // 元々、存在している
        }
      ]
    }
  ];

  console.log(JSON.stringify(rootObject.children)); // [{"id":1,"parent":0,"children":[{"id":3,"parent":1,"children":[{"id":5,"parent":3,"children":[]}]}]},{"id":2,"parent":0,"children":[{"id":4,"parent":2,"children":[]}]}]
  console.log(JSON.stringify(after));               // [{"id":1,"parent":0,"children":[{"id":3,"parent":1,"children":[{"id":5,"parent":3,"children":[]}]}]},{"id":2,"parent":0,"children":[{"id":4,"parent":2,"children":[]}]}]
  console.log(JSON.stringify(rootObject.children) ===...