Sorting a category list from a flat database for insert into a hierarchy-constrained one

sortCategoriesForInsert.js

module.exports = function sortCategoriesForInsert (inputJson) {

   let dataTree = function (inputJson, root) {
        var r = [], o = {};
        inputJson.forEach(function (a) {
            o[a.id] = { data: a, children: o[a.id] && o[a.id].children };
            if (a.parent_id === root) {
                r.push(o[a.id]);
            } else {
                o[a.parent_id] = o[a.parent_id] || {};
                o[a.parent_id].children = o[a.parent_id].children || [];
                o[a.parent_id].children.push(o[a.id]);
            }
        });
        return r;
    }(inputJson, null)

  let sortedDataTree = dataTree.reduce(function traverse(r, a) {
        return r.concat(a.data, (a.children || []).reduce(traverse, []));
  }, [])

  return sortedDataTree
}