Time limit: 45 minutes · React + Tailwind (frontend), Node.js + Express (backend), fs module for storage (no database).

A player enters their name and clicks Start Playing. Their session runs live on the server and they unlock badges the longer they play. Stop Playing ends the session and saves it permanently. Time is compressed: 1 real second = 1 game minute.

Badges must be generated on the server, automatically, as time passes — not on the frontend, and not only when the session stops.

Two terminals:

# Terminal 1 — backend (http://localhost:3001)
cd backend
npm install
npm start

# Terminal 2 — frontend (http://localhost:5173)
cd frontend
npm install
npm run dev

Open http://localhost:5173 — the app will show an error at first, because the states and handlers the UI needs don't exist yet. Creating them is your first task (there are // TODO comments marking every spot). The dev server already proxies /sessions to the backend.

Only two files:

• POST /sessions/start — create an in-memory session, start a 1-second server timer that increments elapsedMinutes and unlocks badges as thresholds are crossed
• POST /sessions/:id/stop — stop the timer, mark "completed", append to data.json with fs
• POST /sessions/:id/add-time — fast-forward an active session; all badges crossed by the jump must unlock (25 → 75 unlocks Bronze, Silver, AND Gold)
• GET /sessions — active sessions (memory) + completed sessions (data.json)

The file has no routes yet — you create all four. Each one has a // TODO comment with step-by-step details, and the in-memory session storage and badge thresholds are provided as commented code you can uncomment.

• Create the playerName and sessions states the JSX expects, and poll GET /sessions every second
• Create the startSession, stopSession, addTime handlers calling the endpoints
• Replace EXAMPLE_SESSIONS with real data, active sessions listed first

Everything else is provided: the Express server bootstrap, the Vite/Tailwind setup, and the fully-styled SessionCard component (badges, MAX indicator, buttons, inputs — just pass it session, onStop, onAddTime).

• Store timer references in a Map keyed by session id — you'll need them later to stop the right timer.
• Think about which sessions belong in memory and which belong in the file.
• Bronze unlocks at 30, not after it.
• If your badge check only tests elapsedMinutes === 30, Add Time will skip badges. Check ranges crossed, not exact values.
• Handle the case where data.json doesn't exist or is empty when reading.

• Badge logic must live on the backend.
• No database, no external storage libraries — only the built-in fs module.
• No edit/delete, authentication, or validation beyond requiring a player name.
• It's fine if active sessions are lost when the server restarts.

What happens to an active session if the server crashes? How would you redesign the timer logic so elapsed time survives a restart?