convert JSON object to JSON tree

Question

var obj = [{
    id: 1,
    child:[2,4],
    data : \"hello\"
},{
    id: 2,
    child:[3],
    data : \"I m second\"
},
{   
    id: 3,
    child:[],
    data : \"I         


        

Answer 1

Well ok.. as i have commented this one was a good question and it was a pleasure to give some thinking over it. Obviously this turns out to be harder than flattening an array of nested objects.

By the way the algorithm doesn't rely on any correlation between the object's ids and the objects keys in the array. An object with any id can be anywhere in the array.

var obj = [{ id: 1, child: [2, 4], data: "hello" }, { id: 2, child: [3], data: "I m second" }, { id: 3, child: [], data: "I m third" }, { id: 4, child: [6], data: "I m fourth" }, { id: 5, child: [], data: "I m fifth" }, { id: 6, child: [], data: "I m sixth" }];

function construct(flat){
  function nest(o) {
    o.forEach( c => { if (!!c.child.length) { // coolness starts here
                         c.child = c.child.map( e => flat.splice(flat.findIndex( f => f.id == e),1)[0]);
                         nest(c.child);
                         }
    	            });
  }
  nest(flat);
  return flat;
}

document.write("<pre>" + JSON.stringify(construct(obj), null, 2) + "</pre>");

Answer 2

A proposal with a temporary object for keeping the reference to the items.

var array = [{ id: 1, child: [2, 4], data: "hello" }, { id: 2, child: [3], data: "I m second" }, { id: 3, child: [], data: "I m third" }, { id: 4, child: [6], data: "I m fourth" }, { id: 5, child: [], data: "I m fifth" }, { id: 6, child: [], data: "I m sixth" }],
    tree = [];

array.forEach(function (a) {
    if (!this[a.id]) {
        this[a.id] = { id: a.id };
        tree.push(this[a.id]);
    }
    this[a.id].data = a.data;
    this[a.id].child = a.child.map(function (b) {
        this[b] = this[b] || { id: b };
        return this[b];
    }, this);
}, Object.create(null));

document.write('<pre>' + JSON.stringify(tree, 0, 4) + '</pre>');