<pre><code id="answer"></code></pre>
	

const entries = [
  {
    index: 1,
    parent: 0
  },
  {
    index: 2,
    parent: 1
  },
  {
    index: 3,
    parent: 2
  },
  {
    index: 4,
    parent: 2
  },
  {
    index: 5,
    parent: 4
  },
  {
    index: 6,
    parent: 5
  },
  {
    index: 7,
    parent: 6
  },
  {
    index: 8,
    parent: 7
  },
  {
    index: 9,
    parent: 8
  },
  {
    index: 10,
    parent: 9
  },
  {
    index: 11,
    parent: 7
  },
  {
    index: 13,
    parent: 11
  },
  {
    index: 12,
    parent: 0
  }
];

const arrayToTree = (arr, parent = 0) =>
arr.filter(item => item.parent === parent)
.map(child => ({ ...child, children: arrayToTree(arr, child.index) }));

const span = document.getElementById("answer");

span.innerHTML = JSON.stringify(arrayToTree(entries),null,2)