81. Comments Thread

Build a Comments Thread component in React that renders nested replies and supports branch-level interactions (reply, collapse/expand) without corrupting tree state.

1. Recursive Rendering

• Render comments as a recursive tree (root -> replies -> deeper replies).
• Each node displays author, timestamp, and body.
• Nested levels should be visually indented.

2. Reply Composer Per Node

• Every comment has a Reply action.
• Clicking Reply opens an inline composer scoped to that comment.
• Submitting inserts a new child under that parent.
• Cancel closes composer without mutating data.

3. Collapse / Expand Branches

• Any comment with children can be collapsed/expanded.
• Collapsing hides descendants only (parent remains visible).
• Collapse state must be keyed by stable comment.id (never array index).

4. Immutable Tree Updates

• Reply insertion must be immutable (no in-place push/splice on existing state).
• Preserve sibling order and untouched branches.
• Multiple inserts at different depths must remain deterministic.

5. Data Contract

interface CommentNode {
  id: string;
  author: string;
  body: string;
  createdAt: string;
  replies?: CommentNode[];
}

interface CommentsThreadProps {
  comments: CommentNode[];
  onChange: (next: CommentNode[]) => void;
}

6. Example Seed Data

const seed: CommentNode[] = [
  {
    id: "c1",
    author: "Ava",
    body: "Great write-up.",
    createdAt: "2026-04-01T09:00:00.000Z",
    replies: [
      {
        id: "c1-1",
        author: "Noah",
        body: "Agreed, especially the caching part.",
        createdAt: "2026-04-01T09:05:00.000Z"
      }
    ]
  }
];