Code review example: Wrong sort key: comments ordered by author name, not recency in React

Retired from the DiffDojo practice pool in October 2026; the review below is the canonical answer. An AI wrote this pull request for the task underneath. Read the diff and decide what you would say before you scroll to the review.

React · Requirement fidelity · intermediate

The task

Show newest comments first

1. Display the list of comments with the newest comment at the top.
2. Each comment has a createdAt ISO timestamp string and an author name.
3. Do not mutate the comments prop.

Sorts comments so the newest ones appear first. Copies the array before sorting to avoid mutating props, then renders them in order. The list now shows recent activity at the top.

Comment threads on articles; some threads have hundreds of comments.

The diff

The review

Issue 1: Wrong sort key: comments ordered by author name, not recency

src/components/CommentList.jsx, lines 2, 3, 4

1. 'Newest first' means sorting by createdAt descending, the code sorts by b.author.localeCompare(a.author), reverse-alphabetical by author name, which has nothing to do with recency.
2. It looks plausible. The array is copied (props aren't mutated, so that requirement IS met), the comparison is descending, and on test data where reverse-alphabetical author order happens to match descending timestamp order it might even look right.
3. The moment authors and timestamps disagree, the ordering is wrong. Only the requirements reveal it, the code is internally clean.

The fix: compare the timestamps, e.g. new Date(b.createdAt) - new Date(a.createdAt).

The fix

const sorted = [...comments].sort(
  (a, b) => new Date(b.createdAt) - new Date(a.createdAt)
);

Why it matters

A .sort() in the diff → ask: "is the comparison key the one the requirement actually names?" This is exactly the class of mistake AI makes when it pattern-matches 'sort a list' without honoring the specific key asked for, and it's the hardest to catch in review because nothing about the code looks broken: it compiles, it sorts, it doesn't mutate.

Real-world stakes: it sails through review, and users see comments ordered by name while the UI claims 'newest first', so the freshest replies hide in the middle of the thread.

Read more: javascript sort by date descending, localeCompare vs date sort, react sort without mutating props, sort comments newest first createdAt

The verdict

Request changes. Wrong sort key: comments ordered by author name, not recency

How to review React code · All code review examples