array_to_tree.js

// idComment, idParentComment, idArticle, userName, userComment, dateAdd
var comments = [
    {
        id: 1,
        parent_id: 0,
        article_id: 1,
        user_name: "Jiffy",
        user_text: "Viva la vida"
    },
    {
        id: 2,
        parent_id: 1,
        article_id: 1,
        user_name: "Lisa",
        user_text: "Me gusta"
    },
    {
        id: 3,
        parent_id: 2,
        article_id: 1,
        user_name: "Romeo",
        user_text: "Ahh say no more"
    },
    {
        id: 4,
        parent_id: 1,
        article_id: 1,
        user_name: "Julia",
        user_text: "I am here"
    },
    {
        id: 5,
        parent_id: 0,
        article_id: 1,
        user_name: "Buffy",
        user_text: "New root comment"
    }
];

// this will create basic structures
function treeInit(comments) {
    /*
        Parent structure:
         { '0': [ 1, 5 ], '1': [ 2, 4 ], '2': [ 3 ] }
     */
    var idParentMap = {};
    /*
        List of comments indexed by ids
        { 1: .., 2: .., ...}
     */
    var commentsObjected = {};

    comments.forEach(function (item) {
        commentsObjected[item.id] = item;

        if (!idParentMap[item.parent_id])
            idParentMap[item.parent_id] = [];

        idParentMap[item.parent_id].push(item.id);
    });

    return {idParentMap, commentsObjected};
}

function treeRecursion(map, comments, layer) {
    var treeLayer = [];

    if(!layer) return [];
    
    // go through this parent and put all its childrens to childs array
    layer.forEach(function (id) {

        comments[id].childs = treeRecursion(map, comments, map[id]);
        treeLayer.push(comments[id]);
    });

    return treeLayer;
}

// create tree structure from flat array
function tree(comments) {
    var tree = [];
    // first go through array and create id map of childs and comment list indexed by comment id
    var {idParentMap, commentsObjected} = treeInit(comments);

    console.log("Id map:", idParentMap);
    console.log("Comments Objected:", commentsObjected);

    // if there is some comment in root of the tree (parent = 0) start recursion
    if(idParentMap[0]) {
        tree = treeRecursion(idParentMap, commentsObjected, idParentMap[0]);
    }

    return tree;
}

// ===============================
var util = require("util");
var c = tree(comments);

console.log(util.inspect(c, false, null));


// outputs:
[nodemon] starting `node tree.js`
Id map: { '0': [ 1, 5 ], '1': [ 2, 4 ], '2': [ 3 ] }
Comments Objected: { '1': 
   { id: 1,
     parent_id: 0,
     article_id: 1,
     user_name: 'Jiffy',
     user_text: 'Viva la vida' },
  '2': 
   { id: 2,
     parent_id: 1,
     article_id: 1,
     user_name: 'Lisa',
     user_text: 'Me gusta' },
  '3': 
   { id: 3,
     parent_id: 2,
     article_id: 1,
     user_name: 'Romeo',
     user_text: 'Ahh say no more' },
  '4': 
   { id: 4,
     parent_id: 1,
     article_id: 1,
     user_name: 'Julia',
     user_text: 'I am here' },
  '5': 
   { id: 5,
     parent_id: 0,
     article_id: 1,
     user_name: 'Buffy',
     user_text: 'New root comment' } }
[ { id: 1,
    parent_id: 0,
    article_id: 1,
    user_name: 'Jiffy',
    user_text: 'Viva la vida',
    childs: 
     [ { id: 2,
         parent_id: 1,
         article_id: 1,
         user_name: 'Lisa',
         user_text: 'Me gusta',
         childs: 
          [ { id: 3,
              parent_id: 2,
              article_id: 1,
              user_name: 'Romeo',
              user_text: 'Ahh say no more',
              childs: [] } ] },
       { id: 4,
         parent_id: 1,
         article_id: 1,
         user_name: 'Julia',
         user_text: 'I am here',
         childs: [] } ] },
  { id: 5,
    parent_id: 0,
    article_id: 1,
    user_name: 'Buffy',
    user_text: 'New root comment',
    childs: [] } ]
[nodemon] clean exit - waiting for changes before restart
[nodemon] restarting due to changes...
[nodemon] starting `node tree.js`
Id map: { '0': [ 1, 5 ], '1': [ 2, 4 ], '2': [ 3 ] }
Comments Objected: { '1': 
   { id: 1,
     parent_id: 0,
     article_id: 1,
     user_name: 'Jiffy',
     user_text: 'Viva la vida' },
  '2': 
   { id: 2,
     parent_id: 1,
     article_id: 1,
     user_name: 'Lisa',
     user_text: 'Me gusta' },
  '3': 
   { id: 3,
     parent_id: 2,
     article_id: 1,
     user_name: 'Romeo',
     user_text: 'Ahh say no more' },
  '4': 
   { id: 4,
     parent_id: 1,
     article_id: 1,
     user_name: 'Julia',
     user_text: 'I am here' },
  '5': 
   { id: 5,
     parent_id: 0,
     article_id: 1,
     user_name: 'Buffy',
     user_text: 'New root comment' } }
[ { id: 1,
    parent_id: 0,
    article_id: 1,
    user_name: 'Jiffy',
    user_text: 'Viva la vida',
    childs: 
     [ { id: 2,
         parent_id: 1,
         article_id: 1,
         user_name: 'Lisa',
         user_text: 'Me gusta',
         childs: 
          [ { id: 3,
              parent_id: 2,
              article_id: 1,
              user_name: 'Romeo',
              user_text: 'Ahh say no more',
              childs: [] } ] },
       { id: 4,
         parent_id: 1,
         article_id: 1,
         user_name: 'Julia',
         user_text: 'I am here',
         childs: [] } ] },
  { id: 5,
    parent_id: 0,
    article_id: 1,
    user_name: 'Buffy',
    user_text: 'New root comment',
    childs: [] } ]