here is one in O(n log n) time treeify

treeify

const flattenArr = [
  { title: '首頁', id: 11, parent_id: 0, children: [] },
  { title: '子頁', id: 12, parent_id: 11, children: [] },
  { title: '產品頁', id: 21, parent_id: 0, children: [] }
]

function treeify(nodes) {
    var indexed_nodes = {}, tree_roots = [];
    for (var i = 0; i < nodes.length; i += 1) {
        indexed_nodes[nodes[i].id] = nodes[i];
    }
    for (var i = 0; i < nodes.length; i += 1) {
        var parent_id = nodes[i].parent_id;
        if (parent_id === 0) {
            tree_roots.push(nodes[i]);
        } else {
            indexed_nodes[parent_id].children.push(nodes[i]);
        }
    }
    return tree_roots;
}

console.log(JSON.stringify(treeify(nodes), undefined, "\t"));