Master React Through Real-World Challenges

Write React and TypeScript in a full Monaco editor with IntelliSense, autocomplete, and optional Vim mode.

Run your code against a real Node + Jest server — the same setup you’d face in production and technical interviews.

Last challenges

#52
30min

Use optimistic

Learn React's useOptimistic hook by building instant heart toggles that update the UI before the server responds — and revert automatically when requests fail.

ddd
useOptimisticstartTransitionreact 19
#53
20min

Best practices

In a technical interview, solving the logic is only half the battle — reviewers also judge your code's quality. A todo list that works perfectly can still hide anti-patterns like derived state in effects, impure renders, or eager initializers that degrade performance. This challenge is different: instead of just checking that your code passes tests, the platform also runs ESLint with React best-practice rules and surfaces every violation. Fix the 3 lint warnings to prove you don't just make it work — you make it right.

ddd
useStateuseEffectESLintbest practices
#54
90min

Spreadsheet

Build a spreadsheet with real-time formula evaluation — a great exercise to practice custom state management, two-way cell bindings, and reusable table components.

ddd
useStateuseEffectcustomHook

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Billed $21quarterly
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