Build a category tree from a flat list

 1// rows: [{ id, parentId, name }, ...]. Top-level rows have parentId = null.
 2function buildTree(rows) {
 3  const roots = [];
 4  for (const row of rows) {
 5    if (row.parentId == null) {
 6      roots.push({ ...row, children: [] });
 7    }
 8  }
 9  function attach(node) {
10    for (const row of rows) {
11      if (row.parentId === node.id) {
12        const child = { ...row, children: [] };
13        node.children.push(child);
14        attach(child);
15      }
16    }
17  }
18  for (const root of roots) attach(root);
19  return roots;
20}