Create tree out of sorted data.

build_tree.js

const source = [
  { Id: "1", Name: "abc", Parent: "", attr: "abc" },
  { Id: "2", Name: "abc", Parent: "1", attr: "abc" },
  { Id: "3", Name: "abc", Parent: "2", attr: "abc" },
  { Id: "4", Name: "abc", Parent: "2", attr: "abc" },
];
function tree(data, id, pId) {
  const [result] = data.reduce(
    ([r, map], item) => {
      const d = { ...item, children: [] };
      const loc = map[item[pId]];
      if (loc) {
        loc.children.push(d);
      } else {
        r.push(d);
      }
      map[item[id]] = d;
      return [r, map];
    },
    [[], {}]
  );
  return result;
}

console.log(JSON.stringify(tree(source, "Id", "Parent"), null, 4));