Snippet to build a nested tree

tree.js

const arr = [
  {id: 1, title: 'hello', parent_id: 0},
  {id: 2, title: 'hello', parent_id: 0},
  {id: 3, title: 'hello', parent_id: 1},
  {id: 4, title: 'hello', parent_id: 3},
  {id: 5, title: 'hello', parent_id: 4},
  {id: 6, title: 'hello', parent_id: 4},
  {id: 7, title: 'hello', parent_id: 3},
  {id: 8, title: 'hello', parent_id: 2}
];

function transformToTree(arr, parentKey = 'parent_id', idKey = 'id'){
  var nodes = {};

  return arr.filter(function(obj){
    var id = obj[idKey],
      parentId = obj[parentKey];

    nodes[id] = Object.assign(obj, nodes[id], { children: [] });
    parentId && (nodes[parentId] = (nodes[parentId] || { children: [] }))["children"].push(obj);

    return !parentId;
  });
}

console.log(JSON.stringify(transformToTree(arr)));

exports = {transformToTree};