Stop grinding
algorithm puzzles.
Start understanding your tools.
Interactive challenges where you re-implement the libraries and patterns you use every day — promises, concurrency limiters, retry logic, event emitters — so you understand the failure modes they solve.
Start Practicing →What top labs actually ask
The best engineering orgs stopped asking you to invert binary trees.
They want you to build a worker queue with concurrency limits, implement
Promise.all from scratch, and demonstrate you understand
runtime behavior — not just surface-level syntax.
Where AI-generated code breaks
LLMs produce async code that looks correct but has subtle bugs around execution ordering, error propagation, and concurrency. If you’ve never built these patterns yourself, you won’t catch them in review — and they’ll ship.
Combat skill atrophy
The more you delegate to AI, the less you exercise the muscle of understanding what your code does. Spend 20 minutes on a challenge, hit the edge cases, keep your fundamentals sharp.
How it works
Each challenge drops you into a code editor with a problem description and a test suite. Write the implementation, run the tests, iterate until everything passes — including the edge cases you didn’t think of. No accounts. No setup. Just you and the code.
Ready to go deeper?
Browse Challenges →