How would you build a file explorer with a nested folder tree?

A file-explorer tree is the canonical "recursive data, recursive component" exercise. The interview signal is in the state shape (don't put expansion state on each node object — keep it lifted) and the tri-state checkbox logic (the tricky part).

Data model. A node is either a folder (has children) or a file. Don't store expanded or checked on the node itself — those are UI state, kept separately so the tree data stays clean and serializable.

type FileNode = { id: string; name: string; type: "file" };
type FolderNode = { id: string; name: string; type: "folder"; children: TreeNode[] };
type TreeNode = FileNode | FolderNode;

Recursive component.

function TreeNodeView({ node, depth = 0 }: { node: TreeNode; depth?: number }) {
  const { expanded, toggleExpanded } = useTreeState();
  const isOpen = expanded.has(node.id);
  return (
    <li role="treeitem" aria-expanded={node.type === "folder" ? isOpen : undefined}>
      <div style={{ paddingLeft: depth * 16 }} onClick={() => node.type === "folder" && toggleExpanded(node.id)}>
        {node.type === "folder" ? (isOpen ? "📂" : "📁") : "📄"} {node.name}
      </div>
      {node.type === "folder" && isOpen && (
        <ul role="group">
          {node.children.map(c => <TreeNodeView key={c.id} node={c} depth={depth + 1} />)}
        </ul>
      )}
    </li>
  );
}

Expansion state — Set<string>, lifted.

function useTreeState() {
  const [expanded, setExpanded] = useState<Set<string>>(new Set());
  const toggleExpanded = useCallback((id: string) => {
    setExpanded(prev => {
      const next = new Set(prev);
      next.has(id) ? next.delete(id) : next.add(id);
      return next;
    });
  }, []);
  return { expanded, toggleExpanded };
}

Lifting to a parent (or a Zustand store) means: collapsed-by-default on every render, "expand all" / "collapse all" is one set update, and persistence is trivial.

Tri-state checkboxes (the hard part). Three checkbox states: checked, unchecked, indeterminate (some descendants are checked). Rules:

• Click a folder → check/uncheck all descendants.
• Click a leaf → propagate up: parent becomes checked if all children checked, indeterminate if some, unchecked if none.

Two approaches:

Approach A — store only leaf state. Keep checked: Set<leafId>. Compute folder state from descendants on render. Pros: no sync bugs. Cons: O(descendants) per folder render — fine for small trees.

function getFolderState(folder: FolderNode, checkedLeaves: Set<string>): "checked" | "unchecked" | "indeterminate" {
  let total = 0, checked = 0;
  function walk(n: TreeNode) {
    if (n.type === "file") { total++; if (checkedLeaves.has(n.id)) checked++; }
    else n.children.forEach(walk);
  }
  walk(folder);
  if (checked === 0) return "unchecked";
  if (checked === total) return "checked";
  return "indeterminate";
}

Approach B — store every node's state. Maintain Map<id, "checked" | "indeterminate" | "unchecked">. On click, walk descendants (set) and ancestors (recompute). Faster reads, more write logic.

For interview: pick A and call out the perf trade-off. For 10k+ nodes, switch to B with memoization.

The DOM checkbox indeterminate property. Not a regular attribute — set it via a ref:

const ref = useRef<HTMLInputElement>(null);
useEffect(() => { if (ref.current) ref.current.indeterminate = state === "indeterminate"; }, [state]);
return <input ref={ref} type="checkbox" checked={state === "checked"} onChange={onToggle} />;

Performance for big trees.

• Memoize TreeNodeView by node id; sibling re-renders don't propagate.
• Virtualize the rendered list — flatten the visible nodes (DFS, skip closed folders) and feed into @tanstack/react-virtual. Tree-virtualization is non-trivial but mandatory at 5k+ nodes.
• Persist expansion to localStorage so reloads don't collapse the user's open state.

Accessibility. Use role="tree" on the root, role="treeitem" on nodes, role="group" on children containers. aria-expanded on folders, aria-selected on current selection. Keyboard: ArrowDown/Up to move, ArrowRight to expand, ArrowLeft to collapse-or-go-to-parent, Enter to activate.

Common variants.

• Lazy-load children: folders fetch on first expand. Cache by id; show inline spinner.
• Drag-and-drop reorder/move: dnd-kit + tree-aware drop zones. Validate (don't drop folder into itself).
• Breadcrumbs: derive from the path of ancestor names of the active node.