Build tree array from flat array in javascript

I have a complex json file that I have to handle with javascript to make it hierarchical, in order to later build a tree. Every entry of the json has : id : a unique id, parentId : the id of the parent node (which is 0 if the node is a root of the tree) level : the level of depth in the tree

The json data is already "ordered". I mean that an entry will have above itself a parent node or brother node, and under itself a child node or a brother node.

Input :

{
    "People": [
        {
            "id": "12",
            "parentId": "0",
            "text": "Man",
            "level": "1",
            "children": null
        },
        {
            "id": "6",
            "parentId": "12",
            "text": "Boy",
            "level": "2",
            "children": null
        },
                {
            "id": "7",
            "parentId": "12",
            "text": "Other",
            "level": "2",
            "children": null
        },
        {
            "id": "9",
            "parentId": "0",
            "text": "Woman",
            "level": "1",
            "children": null
        },
        {
            "id": "11",
            "parentId": "9",
            "text": "Girl",
            "level": "2",
            "children": null
        }
    ],
    "Animals": [
        {
            "id": "5",
            "parentId": "0",
            "text": "Dog",
            "level": "1",
            "children": null
        },
        {
            "id": "8",
            "parentId": "5",
            "text": "Puppy",
            "level": "2",
            "children": null
        },
        {
            "id": "10",
            "parentId": "13",
            "text": "Cat",
            "level": "1",
            "children": null
        },
        {
            "id": "14",
            "parentId": "13",
            "text": "Kitten",
            "level": "2",
            "children": null
        },
    ]
}

Expected output :

{
    "People": [
        {
            "id": "12",
            "parentId": "0",
            "text": "Man",
            "level": "1",
            "children": [
                {
                    "id": "6",
                    "parentId": "12",
                    "text": "Boy",
                    "level": "2",
                    "children": null
                },
                {
                    "id": "7",
                    "parentId": "12",
                    "text": "Other",
                    "level": "2",
                    "children": null
                }   
            ]
        },
        {
            "id": "9",
            "parentId": "0",
            "text": "Woman",
            "level": "1",
            "children":
            {

                "id": "11",
                "parentId": "9",
                "text": "Girl",
                "level": "2",
                "children": null
            }
        }

    ],    

    "Animals": [
        {
            "id": "5",
            "parentId": "0",
            "text": "Dog",
            "level": "1",
            "children": 
                {
                    "id": "8",
                    "parentId": "5",
                    "text": "Puppy",
                    "level": "2",
                    "children": null
                }
        },
        {
            "id": "10",
            "parentId": "13",
            "text": "Cat",
            "level": "1",
            "children": 
            {
                "id": "14",
                "parentId": "13",
                "text": "Kitten",
                "level": "2",
                "children": null
            }
        }

    ]
}

Answers:

Answer

There is an efficient solution if you use a map-lookup. If the parents always come before their children you can merge the two for-loops. It supports multiple roots. It gives an error on dangling branches, but can be modified to ignore them. It doesn't require a 3rd-party library. It's, as far as I can tell, the fastest solution.

function list_to_tree(list) {
    var map = {}, node, roots = [], i;
    for (i = 0; i < list.length; i += 1) {
        map[list[i].id] = i; // initialize the map
        list[i].children = []; // initialize the children
    }
    for (i = 0; i < list.length; i += 1) {
        node = list[i];
        if (node.parentId !== "0") {
            // if you have dangling branches check that map[node.parentId] exists
            list[map[node.parentId]].children.push(node);
        } else {
            roots.push(node);
        }
    }
    return roots;
}

var entries = [
    {
        "id": "12",
        "parentId": "0",
        "text": "Man",
        "level": "1"
    }, { /*...*/ }
];

console.log(list_to_tree(entries));

If you're into complexity theory this solution is ?(n log(n)). The recursive-filter solution is ?(n^2) which can be a problem for large data sets.

Answer

As mentioned by @Sander, @Halcyon`s answer assumes a pre-sorted array, the following does not. (It does however assume you have loaded underscore.js - though it could be written in vanilla javascript):

Code

// Example usage
var arr = [
    {'id':1 ,'parentid' : 0},
    {'id':2 ,'parentid' : 1},
    {'id':3 ,'parentid' : 1},
    {'id':4 ,'parentid' : 2},
    {'id':5 ,'parentid' : 0},
    {'id':6 ,'parentid' : 0},
    {'id':7 ,'parentid' : 4}
];

unflatten = function( array, parent, tree ){
    tree = typeof tree !== 'undefined' ? tree : [];
    parent = typeof parent !== 'undefined' ? parent : { id: 0 };
        
    var children = _.filter( array, function(child){ return child.parentid == parent.id; });
    
    if( !_.isEmpty( children )  ){
        if( parent.id == 0 ){
           tree = children;   
        }else{
           parent['children'] = children
        }
        _.each( children, function( child ){ unflatten( array, child ) } );                    
    }
    
    return tree;
}

tree = unflatten( arr );
document.body.innerHTML = "<pre>" + (JSON.stringify(tree, null, " "))
<script src="https://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.9.1/underscore-min.js"></script>

Requirements

It assumes the properties 'id' and 'parentid' indicate ID and parent ID respectively. There must be elements with parent ID 0, otherwise you get an empty array back. Orphaned elements and their descendants are 'lost'

http://jsfiddle.net/LkkwH/1/

Answer

Had the same problem, but I could not be certain that the data was sorted or not. I could not use a 3rd party library so this is just vanilla Js; Input data can be taken from @Stephen's example;

 var arr = [
        {'id':1 ,'parentid' : 0},
        {'id':4 ,'parentid' : 2},
        {'id':3 ,'parentid' : 1},
        {'id':5 ,'parentid' : 0},
        {'id':6 ,'parentid' : 0},
        {'id':2 ,'parentid' : 1},
        {'id':7 ,'parentid' : 4},
        {'id':8 ,'parentid' : 1}
      ];
    function unflatten(arr) {
      var tree = [],
          mappedArr = {},
          arrElem,
          mappedElem;

      // First map the nodes of the array to an object -> create a hash table.
      for(var i = 0, len = arr.length; i < len; i++) {
        arrElem = arr[i];
        mappedArr[arrElem.id] = arrElem;
        mappedArr[arrElem.id]['children'] = [];
      }


      for (var id in mappedArr) {
        if (mappedArr.hasOwnProperty(id)) {
          mappedElem = mappedArr[id];
          // If the element is not at the root level, add it to its parent array of children.
          if (mappedElem.parentid) {
            mappedArr[mappedElem['parentid']]['children'].push(mappedElem);
          }
          // If the element is at the root level, add it to first level elements array.
          else {
            tree.push(mappedElem);
          }
        }
      }
      return tree;
    }

var tree = unflatten(arr);
document.body.innerHTML = "<pre>" + (JSON.stringify(tree, null, " "))

JS Fiddle

Flat Array to Tree

Answer

Very straightforward way to accomplish is

( BONUS1 : NODES MAY or MAY NOT BE ORDERED )

( BONUS2 : NO 3RD PARTY LIBRARY NEEDED, PLAIN JS )

( BONUS3 : User "Elias Rabl" says this is the fastest solution, see his answer below )

const createDataTree = dataset => {
    let hashTable = Object.create(null)
    dataset.forEach( aData => hashTable[aData.ID] = { ...aData, childNodes : [] } )
    let dataTree = []
    dataset.forEach( aData => {
      if( aData.parentID ) hashTable[aData.parentID].childNodes.push(hashTable[aData.ID])
      else dataTree.push(hashTable[aData.ID])
    } )
    return dataTree
}

Here is a test, it might help you to understand how the solution works :

it('creates a correct shape of dataTree', () => {

    let dataSet = [
        {
            "ID": 1,
            "Phone": "(403) 125-2552",
            "City": "Coevorden",
            "Name": "Grady"
        },
        {
            "ID": 2,
            "parentID": 1,
            "Phone": "(979) 486-1932",
            "City": "Che?m",
            "Name": "Scarlet"
        }
    ]

    let expectedDataTree = [ 
    {
            "ID": 1,
            "Phone": "(403) 125-2552",
            "City": "Coevorden",
            "Name": "Grady",
            childNodes : [
                {
                    "ID": 2,
                    "parentID": 1,
                    "Phone": "(979) 486-1932",
                    "City": "Che?m",
                    "Name": "Scarlet",
                    childNodes : []
                }
            ]
    } 
    ]

  expect( createDataTree(dataSet) ).toEqual(expectedDataTree)
});
Answer

Use this ES6 approach. Works like charm

// Data Set
// One top level comment 
const comments = [{
    id: 1,
    parent_id: null
}, {
    id: 2,
    parent_id: 1
}, {
    id: 3,
    parent_id: 1
}, {
    id: 4,
    parent_id: 2
}, {
    id: 5,
    parent_id: 4
}];

const nest = (items, id = null, link = 'parent_id') =>
  items
    .filter(item => item[link] === id)
    .map(item => ({ ...item, children: nest(items, item.id) }));

console.log(
  nest(comments)
)

Answer

a more simple function list-to-tree-lite

npm install list-to-tree-lite

listToTree(list)

source:

function listToTree(data, options) {
    options = options || {};
    var ID_KEY = options.idKey || 'id';
    var PARENT_KEY = options.parentKey || 'parent';
    var CHILDREN_KEY = options.childrenKey || 'children';

    var tree = [],
        childrenOf = {};
    var item, id, parentId;

    for (var i = 0, length = data.length; i < length; i++) {
        item = data[i];
        id = item[ID_KEY];
        parentId = item[PARENT_KEY] || 0;
        // every item may have children
        childrenOf[id] = childrenOf[id] || [];
        // init its children
        item[CHILDREN_KEY] = childrenOf[id];
        if (parentId != 0) {
            // init its parent's children object
            childrenOf[parentId] = childrenOf[parentId] || [];
            // push it into its parent's children object
            childrenOf[parentId].push(item);
        } else {
            tree.push(item);
        }
    };

    return tree;
}

jsfiddle

Answer

You can handle this question with just two line coding:

_(flatArray).forEach(f=>
           {f.nodes=_(flatArray).filter(g=>g.parentId==f.id).value();});

var resultArray=_(flatArray).filter(f=>f.parentId==null).value();

Test Online (see the browser console for created tree)

Requirements:

1- Install lodash 4 (a Javascript library for manipulating objects and collections with performant methods => like the Linq in c#) Lodash

2- A flatArray like below:

    var flatArray=
    [{
      id:1,parentId:null,text:"parent1",nodes:[]
    }
   ,{
      id:2,parentId:null,text:"parent2",nodes:[]
    }
    ,
    {
      id:3,parentId:1,text:"childId3Parent1",nodes:[]
    }
    ,
    {
      id:4,parentId:1,text:"childId4Parent1",nodes:[]
    }
    ,
    {
      id:5,parentId:2,text:"childId5Parent2",nodes:[]
    }
    ,
    {
      id:6,parentId:2,text:"childId6Parent2",nodes:[]
    }
    ,
    {
      id:7,parentId:3,text:"childId7Parent3",nodes:[]
    }
    ,
    {
      id:8,parentId:5,text:"childId8Parent5",nodes:[]
    }];

Thank Mr. Bakhshabadi

Good luck

Answer

It may be useful package list-to-tree Install:

bower install list-to-tree --save

or

npm install list-to-tree --save

For example, have list:

var list = [
  {
    id: 1,
    parent: 0
  }, {
    id: 2,
    parent: 1
  }, {
    id: 3,
    parent: 1
  }, {
    id: 4,
    parent: 2
  }, {
    id: 5,
    parent: 2
  }, {
    id: 6,
    parent: 0
  }, {
    id: 7,
    parent: 0
  }, {
    id: 8,
    parent: 7
  }, {
    id: 9,
    parent: 8
  }, {
    id: 10,
    parent: 0
  }
];

Use package list-to-tree:

var ltt = new LTT(list, {
  key_id: 'id',
  key_parent: 'parent'
});
var tree = ltt.GetTree();

Result:

[{
  "id": 1,
  "parent": 0,
  "child": [
    {
      "id": 2,
      "parent": 1,
      "child": [
        {
          "id": 4,
          "parent": 2
        }, {
          "id": 5, "parent": 2
        }
      ]
    },
    {
      "id": 3,
      "parent": 1
    }
  ]
}, {
  "id": 6,
  "parent": 0
}, {
  "id": 7,
  "parent": 0,
  "child": [
    {
      "id": 8,
      "parent": 7,
      "child": [
        {
          "id": 9,
          "parent": 8
        }
      ]
    }
  ]
}, {
  "id": 10,
  "parent": 0
}];
Answer

I've written a test script to evaluate the performance of the two most general solutions (meaning that the input does not have to be sorted beforehand and that the code does not depend on third party libraries), proposed by users shekhardtu (see answer) and FurkanO (see answer).

http://playcode.io/316025?tabs=console&script.js&output

FurkanO's solution seems to be the fastest.

Answer

This is a proposal for unordered items. This function works with a single loop and with a hash table and collects all items with their id. If a root node is found, then the object is added to the result array.

function getTree(data, root) {
    var o = {};
    data.forEach(function (a) {
        if (o[a.id] && o[a.id].children) {
            a.children = o[a.id].children;
        }
        o[a.id] = a;
        o[a.parentId] = o[a.parentId] || {};
        o[a.parentId].children = o[a.parentId].children || [];
        o[a.parentId].children.push(a);
    });
    return o[root].children;
}

var data = { People: [{ id: "12", parentId: "0", text: "Man", level: "1", children: null }, { id: "6", parentId: "12", text: "Boy", level: "2", children: null }, { id: "7", parentId: "12", text: "Other", level: "2", children: null }, { id: "9", parentId: "0", text: "Woman", level: "1", children: null }, { id: "11", parentId: "9", text: "Girl", level: "2", children: null }], Animals: [{ id: "5", parentId: "0", text: "Dog", level: "1", children: null }, { id: "8", parentId: "5", text: "Puppy", level: "2", children: null }, { id: "10", parentId: "13", text: "Cat", level: "1", children: null }, { id: "14", parentId: "13", text: "Kitten", level: "2", children: null }] },
    tree = Object.keys(data).reduce(function (r, k) {
        r[k] = getTree(data[k], '0');
        return r;
    }, {});

console.log(tree);
.as-console-wrapper { max-height: 100% !important; top: 0; }

Answer

Here's a simple helper function that I created modeled after the above answers, tailored to a Babel environment:

import { isEmpty } from 'lodash'

export default function unflattenEntities(entities, parent = {id: null}, tree = []) {

  let children = entities.filter( entity => entity.parent_id == parent.id)

  if (!isEmpty( children )) {
    if ( parent.id == null ) {
      tree = children
    } else {
      parent['children'] = children
    }
    children.map( child => unflattenEntities( entities, child ) )
  }

  return tree

}
Answer

also do it with lodashjs(v4.x)

function buildTree(arr){
  var a=_.keyBy(arr, 'id')
  return _
   .chain(arr)
   .groupBy('parentId')
   .forEach(function(v,k){ 
     k!='0' && (a[k].children=(a[k].children||[]).concat(v));
   })
   .result('0')
   .value();
}
Answer

I like @WilliamLeung's pure JavaScript solution, but sometimes you need to make changes in existing array to keep a reference to object.

function listToTree(data, options) {
  options = options || {};
  var ID_KEY = options.idKey || 'id';
  var PARENT_KEY = options.parentKey || 'parent';
  var CHILDREN_KEY = options.childrenKey || 'children';

  var item, id, parentId;
  var map = {};
    for(var i = 0; i < data.length; i++ ) { // make cache
    if(data[i][ID_KEY]){
      map[data[i][ID_KEY]] = data[i];
      data[i][CHILDREN_KEY] = [];
    }
  }
  for (var i = 0; i < data.length; i++) {
    if(data[i][PARENT_KEY]) { // is a child
      if(map[data[i][PARENT_KEY]]) // for dirty data
      {
        map[data[i][PARENT_KEY]][CHILDREN_KEY].push(data[i]); // add child to parent
        data.splice( i, 1 ); // remove from root
        i--; // iterator correction
      } else {
        data[i][PARENT_KEY] = 0; // clean dirty data
      }
    }
  };
  return data;
}

Exapmle: https://jsfiddle.net/kqw1qsf0/17/

Answer

var data = [{"country":"india","gender":"male","type":"lower","class":"X"},
			{"country":"china","gender":"female","type":"upper"},
			{"country":"india","gender":"female","type":"lower"},
			{"country":"india","gender":"female","type":"upper"}];
var seq = ["country","type","gender","class"];
var treeData = createHieArr(data,seq);
console.log(treeData)
function createHieArr(data,seq){
	var hieObj = createHieobj(data,seq,0),
		hieArr = convertToHieArr(hieObj,"Top Level");
		return [{"name": "Top Level", "parent": "null",
				     "children" : hieArr}]
	function convertToHieArr(eachObj,parent){
		var arr = [];
		for(var i in eachObj){
			arr.push({"name":i,"parent":parent,"children":convertToHieArr(eachObj[i],i)})
		}
		return arr;
	}
	function createHieobj(data,seq,ind){
		var s = seq[ind];
		if(s == undefined){
			return [];
		}
		var childObj = {};
		for(var ele of data){
			if(ele[s] != undefined){
				if(childObj[ele[s]] == undefined){
					childObj[ele[s]] = [];
				}
				childObj[ele[s]].push(ele);
			}
		}
		ind = ind+1;
		for(var ch in childObj){
			childObj[ch] = createHieobj(childObj[ch],seq,ind)
		}
		return childObj;
	}
}

Answer

You might have a look at the npm package tree-util It can also be very handy if you want to load data from an SQL DB data table. You can also easily add extra data to the nodes in the created tree.

Disclaimer, I made this package.

Answer

You can use this "treeify" package from Github here or NPM.

Installation:

$ npm install --save-dev treeify-js

Answer

I had similar issue couple days ago when have to display folder tree from flat array. I didn't see any solution in TypeScript here so I hope it will be helpful.

In my cases main parent were only one, also rawData array don't have to be sorted. Solutions base on prepare temp object like {parentId: [child1, child2, ...] }

example raw data

const flatData: any[] = Folder.ofCollection([
  {id: '1', title: 'some title' },
  {id: '2', title: 'some title', parentId: 1 },
  {id: '3', title: 'some title', parentId: 7 },
  {id: '4', title: 'some title', parentId: 1 },
  {id: '5', title: 'some title', parentId: 2 },
  {id: '6', title: 'some title', parentId: 5 },
  {id: '7', title: 'some title', parentId: 5 },

]);

def of Folder

export default class Folder {
    public static of(data: any): Folder {
        return new Folder(data);
    }

    public static ofCollection(objects: any[] = []): Folder[] {
        return objects.map((obj) => new Folder(obj));
    }

    public id: string;
    public parentId: string | null;
    public title: string;
    public children: Folder[];

    constructor(data: any = {}) {
        this.id = data.id;
        this.parentId = data.parentId || null;
        this.title = data.title;
        this.children = data.children || [];
    }
}

SOLUTION: Function that returns tree structure for flat argument

    public getTree(flatData: any[]): Folder[] {
        const addChildren = (item: Folder) => {
            item.children = tempChild[item.id] || [];
            if (item.children.length) {
                item.children.forEach((child: Folder) => {
                    addChildren(child);
                });
            }
        };

        const tempChild: any = {};
        flatData.forEach((item: Folder) => {
            const parentId = item.parentId || 0;
            Array.isArray(tempChild[parentId]) ? tempChild[parentId].push(item) : (tempChild[parentId] = [item]);
        });

        const tree: Folder[] = tempChild[0];
        tree.forEach((base: Folder) => {
            addChildren(base);
        });
        return tree;
    }
Answer

Here is a modified version of Steven Harris' that is plain ES5 and returns an object keyed on the id rather than returning an array of nodes at both the top level and for the children.

unflattenToObject = function(array, parent) {
  var tree = {};
  parent = typeof parent !== 'undefined' ? parent : {id: 0};

  var childrenArray = array.filter(function(child) {
    return child.parentid == parent.id;
  });

  if (childrenArray.length > 0) {
    var childrenObject = {};
    // Transform children into a hash/object keyed on token
    childrenArray.forEach(function(child) {
      childrenObject[child.id] = child;
    });
    if (parent.id == 0) {
      tree = childrenObject;
    } else {
      parent['children'] = childrenObject;
    }
    childrenArray.forEach(function(child) {
      unflattenToObject(array, child);
    })
  }

  return tree;
};

var arr = [
    {'id':1 ,'parentid': 0},
    {'id':2 ,'parentid': 1},
    {'id':3 ,'parentid': 1},
    {'id':4 ,'parentid': 2},
    {'id':5 ,'parentid': 0},
    {'id':6 ,'parentid': 0},
    {'id':7 ,'parentid': 4}
];
tree = unflattenToObject(arr);
Answer

This is a modified version of the above that works with multiple root items, I use GUIDs for my ids and parentIds so in the UI that creates them I hard code root items to something like 0000000-00000-00000-TREE-ROOT-ITEM

var tree = unflatten(records, "TREE-ROOT-ITEM");

function unflatten(records, rootCategoryId, parent, tree){
    if(!_.isArray(tree)){
        tree = [];
        _.each(records, function(rec){
            if(rec.parentId.indexOf(rootCategoryId)>=0){        // change this line to compare a root id
            //if(rec.parentId == 0 || rec.parentId == null){    // example for 0 or null
                var tmp = angular.copy(rec);
                tmp.children = _.filter(records, function(r){
                    return r.parentId == tmp.id;
                });
                tree.push(tmp);
                //console.log(tree);
                _.each(tmp.children, function(child){
                    return unflatten(records, rootCategoryId, child, tree);
                });
            }
        });
    }
    else{
        if(parent){
            parent.children = _.filter(records, function(r){
                return r.parentId == parent.id;
            });
            _.each(parent.children, function(child){
                return unflatten(records, rootCategoryId, child, tree);
            });
        }
    }
    return tree;
}
Answer

Copied from the Internet http://jsfiddle.net/stywell/k9x2a3g6/

    function list2tree(data, opt) {
        opt = opt || {};
        var KEY_ID = opt.key_id || 'ID';
        var KEY_PARENT = opt.key_parent || 'FatherID';
        var KEY_CHILD = opt.key_child || 'children';
        var EMPTY_CHILDREN = opt.empty_children;
        var ROOT_ID = opt.root_id || 0;
        var MAP = opt.map || {};
        function getNode(id) {
            var node = []
            for (var i = 0; i < data.length; i++) {
                if (data[i][KEY_PARENT] == id) {
                    for (var k in MAP) {
                        data[i][k] = data[i][MAP[k]];
                    }
                    if (getNode(data[i][KEY_ID]) !== undefined) {
                        data[i][KEY_CHILD] = getNode(data[i][KEY_ID]);
                    } else {
                        if (EMPTY_CHILDREN === null) {
                            data[i][KEY_CHILD] = null;
                        } else if (JSON.stringify(EMPTY_CHILDREN) === '[]') {
                            data[i][KEY_CHILD] = [];
                        }
                    }
                    node.push(data[i]);
                }
            }
            if (node.length == 0) {
                return;
            } else {
                return node;
            }
        }
        return getNode(ROOT_ID)
    }

    var opt = {
        "key_id": "ID",              //???ID
        "key_parent": "FatherID",    //?????ID
        "key_child": "children",     //??????
        "empty_children": [],        //??????,????  //???????,??????????key_child??;????null??[],??
        "root_id": 0,                //??????ID
        "map": {                     //?????????  //???????????; ???????????,???????
            "value": "ID",
            "label": "TypeName",
        }
    };
Answer

You can use npm package array-to-tree https://github.com/alferov/array-to-tree. It's convert a plain array of nodes (with pointers to parent nodes) to a nested data structure.

Solves a problem with conversion of retrieved from a database sets of data to a nested data structure (i.e. navigation tree).

Usage:

var arrayToTree = require('array-to-tree');

var dataOne = [
  {
    id: 1,
    name: 'Portfolio',
    parent_id: undefined
  },
  {
    id: 2,
    name: 'Web Development',
    parent_id: 1
  },
  {
    id: 3,
    name: 'Recent Works',
    parent_id: 2
  },
  {
    id: 4,
    name: 'About Me',
    parent_id: undefined
  }
];

arrayToTree(dataOne);

/*
 * Output:
 *
 * Portfolio
 *   Web Development
 *     Recent Works
 * About Me
 */
Answer

This is an old thread but I figured an update never hurts, with ES6 you can do:

(parent => categories.filter(cat => cat. parentId === parent).map(cat=>({...cat, children: recursive1(cat.id)})))(0)

hope it helps someone

Answer

this is what i used in a react project

// ListToTree.js
import _filter from 'lodash/filter';
import _map from 'lodash/map';

export default (arr, parentIdKey) => _map(_filter(arr, ar => !ar[parentIdKey]), ar => ({
  ...ar,
  children: _filter(arr, { [parentIdKey]: ar.id }),
}));

usage:

// somewhere.js
import ListToTree from '../Transforms/ListToTree';

const arr = [
   {
      "id":"Bci6XhCLZKPXZMUztm1R",
      "name":"Sith"
   },
   {
      "id":"C3D71CMmASiR6FfDPlEy",
      "name":"Luke",
      "parentCategoryId":"ltatOlEkHdVPf49ACCMc"
   },
   {
      "id":"aS8Ag1BQqxkO6iWBFnsf",
      "name":"Obi Wan",
      "parentCategoryId":"ltatOlEkHdVPf49ACCMc"
   },
   {
      "id":"ltatOlEkHdVPf49ACCMc",
      "name":"Jedi"
   },
   {
      "id":"pw3CNdNhnbuxhPar6nOP",
      "name":"Palpatine",
      "parentCategoryId":"Bci6XhCLZKPXZMUztm1R"
   }
];
const response = ListToTree(arr, 'parentCategoryId');

output:

[
   {
      "id":"Bci6XhCLZKPXZMUztm1R",
      "name":"Sith",
      "children":[
         {
            "id":"pw3CNdNhnbuxhPar6nOP",
            "name":"Palpatine",
            "parentCategoryId":"Bci6XhCLZKPXZMUztm1R"
         }
      ]
   },
   {
      "id":"ltatOlEkHdVPf49ACCMc",
      "name":"Jedi",
      "children":[
         {
            "id":"C3D71CMmASiR6FfDPlEy",
            "name":"Luke",
            "parentCategoryId":"ltatOlEkHdVPf49ACCMc"
         },
         {
            "id":"aS8Ag1BQqxkO6iWBFnsf",
            "name":"Obi Wan",
            "parentCategoryId":"ltatOlEkHdVPf49ACCMc"
         }
      ]
   }
]```
Answer
  1. without third party library
  2. no need for pre-ordering array
  3. you can get any portion of the tree you want

Try this

function getUnflatten(arr,parentid){
  let output = []
  for(const obj of arr){
    if(obj.parentid == parentid)

      let children = getUnflatten(arr,obj.id)

      if(children.length){
        obj.children = children
      }
      output.push(obj)
    }
  }

  return output
 }

Test it on Jsfiddle

Answer

My typescript solution, maybe it helps you:

type ITreeItem<T> = T & {
    children: ITreeItem<T>[],
};

type IItemKey = string | number;

function createTree<T>(
    flatList: T[],
    idKey: IItemKey,
    parentKey: IItemKey,
): ITreeItem<T>[] {
    const tree: ITreeItem<T>[] = [];

    // hash table.
    const mappedArr = {};
    flatList.forEach(el => {
        const elId: IItemKey = el[idKey];

        mappedArr[elId] = el;
        mappedArr[elId].children = [];
    });

    // also you can use Object.values(mappedArr).forEach(...
    // but if you have element which was nested more than one time
    // you should iterate flatList again:
    flatList.forEach((elem: ITreeItem<T>) => {
        const mappedElem = mappedArr[elem[idKey]];

        if (elem[parentKey]) {
            mappedArr[elem[parentKey]].children.push(elem);
        } else {
            tree.push(mappedElem);
        }
    });

    return tree;
}

Example of usage:

createTree(yourListData, 'id', 'parentId');
Answer

I wrote an ES6 version based on @Halcyon answer

const array = [
  {
    id: '12',
    parentId: '0',
    text: 'one-1'
  },
  {
    id: '6',
    parentId: '12',
    text: 'one-1-6'
  },
  {
    id: '7',
    parentId: '12',
    text: 'one-1-7'
  },

  {
    id: '9',
    parentId: '0',
    text: 'one-2'
  },
  {
    id: '11',
    parentId: '9',
    text: 'one-2-11'
  }
];

// Prevent changes to the original data
const arrayCopy = array.map(item => ({ ...item }));

const listToTree = list => {
  const map = {};
  const roots = [];

  list.forEach((v, i) => {
    map[v.id] = i;
    list[i].children = [];
  });

  list.forEach(v => (v.parentId !== '0' ? list[map[v.parentId]].children.push(v) : roots.push(v)));

  return roots;
};

console.log(listToTree(arrayCopy));

The principle of this algorithm is to use "map" to establish an index relationship. It is easy to find "item" in the list by "parentId", and add "children" to each "item", because "list" is a reference relationship, so "roots" will Build relationships with the entire tree.

Answer

Convert nodes Array to Tree

ES6 function to convert an Array of nodes (related by parent ID) - to a Tree structure:

/**
 * Convert nodes list related by parent ID - to tree.
 * @syntax getTree(nodesArray [, rootID [, propertyName]])
 *
 * @param {Array} arr   Array of nodes
 * @param {integer} id  Defaults to 0
 * @param {string} p    Property name. Defaults to "parent_id"
 * @returns {Object}    Nodes tree
 */

const getTree = (arr, p = "parent_id") => arr.reduce((o, n) => {

  if (!o[n.id]) o[n.id] = {};
  if (!o[n[p]]) o[n[p]] = {};
  if (!o[n[p]].nodes) o[n[p]].nodes= [];
  if (o[n.id].nodes) n.nodes= o[n.id].nodes;

  o[n[p]].nodes.push(n);
  o[n.id] = n;

  return o;
}, {});

Generate HTML List from nodes Tree

Having our Tree in place, here's a recursive function to build the UL > LI Elements:

/**
 * Convert Tree structure to UL>LI and append to Element
 * @syntax getTree(treeArray [, TargetElement [, onLICreatedCallback ]])
 *
 * @param {Array} tree Tree array of nodes
 * @param {Element} el HTMLElement to insert into
 * @param {function} cb Callback function called on every LI creation
 */

const treeToHTML = (tree, el, cb) => el.append(tree.reduce((ul, n) => {
  const li = document.createElement('li');

  if (cb) cb.call(li, n);
  if (n.nodes?.length) treeToHTML(n.nodes, li, cb);

  ul.append(li);
  return ul;
}, document.createElement('ul')));

Demo time

Here's an example having a linear Array of nodes and using both the above functions:

const getTree = (arr, p = "parent_id") => arr.reduce((o, n) => {
  if (!o[n.id]) o[n.id] = {};
  if (!o[n[p]]) o[n[p]] = {};
  if (!o[n[p]].nodes) o[n[p]].nodes = [];
  if (o[n.id].nodes) n.nodes = o[n.id].nodes;
  o[n[p]].nodes.push(n);
  o[n.id] = n;
  return o;
}, {});


const treeToHTML = (tree, el, cb) => el.append(tree.reduce((ul, n) => {
  const li = document.createElement('li');
  if (cb) cb.call(li, n);
  if (n.nodes?.length) treeToHTML(n.nodes, li, cb);
  ul.append(li);
  return ul;
}, document.createElement('ul')));


// DEMO TIME:

const nodesList = [
  {id: 10,  parent_id: 4,  text: "Item 10"}, // PS: Order does not matters
  {id: 1,   parent_id: 0,  text: "Item 1"},  
  {id: 4,   parent_id: 0,  text: "Item 4"},
  {id: 3,   parent_id: 5,  text: "Item 3"},
  {id: 5,   parent_id: 4,  text: "Item 5"},
  {id: 2,   parent_id: 1,  text: "Item 2"},
];
const myTree = getTree(nodesList)[0].nodes; // Get nodes of Root (0)

treeToHTML(myTree, document.querySelector("#tree"), function(node) {
  this.textContent = `(${node.parent_id} ${node.id}) ${node.text}`;
  this._node = node;
  this.addEventListener('click', clickHandler);
});

function clickHandler(ev) {
  if (ev.target !== this) return;
  console.clear();
  console.log(this._node.id);
};
<div id="tree"></div>

Answer

Answer to a similar question:

https://stackoverflow.com/a/61575152/7388356

UPDATE

You can use Map object introduced in ES6. Basically instead of finding parents by iterating over the array again, you'll just get the parent item from the array by parent's id like you get items in an array by index.

Here is the simple example:

const people = [
  {
    id: "12",
    parentId: "0",
    text: "Man",
    level: "1",
    children: null
  },
  {
    id: "6",
    parentId: "12",
    text: "Boy",
    level: "2",
    children: null
  },
  {
    id: "7",
    parentId: "12",
    text: "Other",
    level: "2",
    children: null
  },
  {
    id: "9",
    parentId: "0",
    text: "Woman",
    level: "1",
    children: null
  },
  {
    id: "11",
    parentId: "9",
    text: "Girl",
    level: "2",
    children: null
  }
];

function toTree(arr) {
  let arrMap = new Map(arr.map(item => [item.id, item]));
  let tree = [];

  for (let i = 0; i < arr.length; i++) {
    let item = arr[i];

    if (item.parentId !== "0") {
      let parentItem = arrMap.get(item.parentId);

      if (parentItem) {
        let { children } = parentItem;

        if (children) {
          parentItem.children.push(item);
        } else {
          parentItem.children = [item];
        }
      }
    } else {
      tree.push(item);
    }
  }

  return tree;
}

let tree = toTree(people);

console.log(tree);

Edit crazy-williams-glgj3

Answer

Based on @FurkanO's answer, I created another version that does not mutate the origial data (like @Dac0d3r requested). I really liked @shekhardtu's answer, but realized it had to filter through the data many times. I thought a solution could be to use FurkanO's answer by copying the data first. I tried my version in jsperf, and the results where unfortunately (very) bleak... It seems like the accepted answer is really a good one! My version is quite configurable and failsafe though, so I share it with you guys anyway; here is my contribution:

function unflat(data, options = {}) {
    const { id, parentId, childrenKey } = {
        id: "id",
        parentId: "parentId",
        childrenKey: "children",
        ...options
    };
    const copiesById = data.reduce(
        (copies, datum) => ((copies[datum[id]] = datum) && copies),
        {}
    );
    return Object.values(copiesById).reduce(
        (root, datum) => {
            if ( datum[parentId] && copiesById[datum[parentId]] ) {
                copiesById[datum[parentId]][childrenKey] = [ ...copiesById[datum[parentId]][childrenKey], datum ];
            } else {
                root = [ ...root, datum ];
            }
            return root
        }, []
    );
}

const data = [
    {
        "account": "10",
        "name": "Konto 10",
        "parentAccount": null
    },{
        "account": "1010",
        "name": "Konto 1010",
        "parentAccount": "10"
    },{
        "account": "10101",
        "name": "Konto 10101",
        "parentAccount": "1010"
    },{
        "account": "10102",
        "name": "Konto 10102",
        "parentAccount": "1010"
    },{
        "account": "10103",
        "name": "Konto 10103",
        "parentAccount": "1010"
    },{
        "account": "20",
        "name": "Konto 20",
        "parentAccount": null
    },{
        "account": "2020",
        "name": "Konto 2020",
        "parentAccount": "20"
    },{
        "account": "20201",
        "name": "Konto 20201",
        "parentAccount": "2020"
    },{
        "account": "20202",
        "name": "Konto 20202",
        "parentAccount": "2020"
    }
];

const options = {
    id: "account",
    parentId: "parentAccount",
    childrenKey: "children"
};

console.log(
    "Hierarchical tree",
    unflat(data, options)
);

With the options parameter, it is possible to configure what property to use as id or parent id. It is also possible to configure the name of the children property, if someone wants "childNodes": [] or something.

OP could simply use default options:

input.People = unflat(input.People);

If the parent id is falsy (null, undefined or other falsy values) or the parent object does not exist, we consider the object to be a root node.

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