codigo_categoria.ts

    this.api.get('categories')
      .subscribe(data => {
        this.categories = data
        var indexed_nodes = {}, tree_roots = [];
        for (var k
           = 0; k < data.length; k += 1) {
            data[k].children = [];            
        }
        for (var i = 0; i < data.length; i += 1) {
            indexed_nodes[data[i]._id] = data[i];
            
        }
        for (var j = 0; j < data.length; j += 1) {
            var parent = data[j].parent;
            if (parent === undefined) {
                tree_roots.push(data[j]);
            } else {
                indexed_nodes[parent].children.push(data[j]);
            }
        }
        console.log(JSON.stringify(tree_roots, undefined, "\t"));
      });
  }