Coding practice drills

PrepAI's coding practice is a library of self-contained "drills" organised into language tracks, where each drill is a complete mini-problem — statement, expected output, starter code, hidden tests, layered hints, a reference solution, and complexity notes — that you solve in an in-browser editor. Your code runs locally in a sandboxed Web Worker and is checked automatically against the drill's tests.

Language tracks

Drills are grouped into tracks. Each track is a curated progression from fundamentals to interview-grade problems:

Track	Focus	Status
JavaScript	Core language plus 140 Web-API drills (DOM, fetch, storage, workers, etc.)	Shipped
TypeScript	Generics, decorators, and interview classics	Shipped
Node.js	Standard library, HTTP, system primitives, protocols, advanced systems	Shipped
Node microservices	RPC, pub/sub, gateways, observability, distributed consensus	Shipped
HTML	Foundations plus forms, tables, lists, media, accessibility	In progress
CSS	Layout, the box model, selectors, grid	In progress
Python · AWS · Firebase	Planned	Marked "Coming soon" on the tracks hub

Tracks that are still being authored are shown honestly as Coming soon on the hub so you always know what is and isn't available. Hundreds of drills are already live across the shipped tracks.

Anatomy of a drill

Every drill is deliberately self-contained so you can practise one without depending on another:

1. Problem statement — what to build, in plain language.
2. Expected output examples — concrete input → output pairs.
3. Starter code — a function signature with a // TODO so you know exactly what to implement.
4. Hidden tests — typically six to eight assertSource assertions that grade your solution.
5. Layered hints — progressively more revealing nudges.
6. Three-tier guide — short, medium, and deep explanations.
7. Reference solution — the canonical answer.
8. Complexity & follow-ups — Big-O analysis and "what would the interviewer ask next?".

The in-browser runner

When you press Run, PrepAI executes your code in a Web Worker — a background thread isolated from the page. This has three benefits:

• Privacy: your code never leaves your device. There is no server-side compiler.
• Safety: a runaway loop can't freeze the UI, and a time budget stops infinite loops.
• Speed: results come back immediately, with per-test pass/fail and the actual vs expected values.

JavaScript and TypeScript share the same worker; other languages plug in their own adapters over time.

Editors

You can choose your editor and the choice persists across sessions (stored on-device):

• CodeMirror 6 — the default, lightweight and fast.
• Monaco — the VS Code editor, opt-in from settings, with richer IntelliSense.

Progress tracking

Completing a drill records progress to your account, keyed per user and topic. That feeds your streak, per-track completion, and milestones on the dashboard, and it determines when the reference solution unlocks (after at least one real attempt on the free plan).

Tips for effective practice

• Attempt before revealing. The solution unlocks after a genuine attempt for a reason — retrieval beats recognition.
• Read the expected outputs first. Most failing cases come from missed edge conditions (empty input, negatives, duplicates).
• Do a track in order. Tracks are sequenced from fundamentals upward; skipping around leaves gaps.
• Note the follow-ups. Interviewers rarely stop at the first solution — the follow-up prompts mirror that.

FAQ

Are the drills free? Yes. Unlimited topic viewing and in-browser runs are on the free plan. Paid plans add download conveniences — see Subscription plans.

Does my code get uploaded? No. Execution happens in a Web Worker on your device; only your completion status is saved.

Which languages are supported today? JavaScript, TypeScript, Node.js, and Node microservices are fully shipped, with HTML and CSS in progress. Python, AWS, and Firebase are planned and clearly marked "Coming soon".

Can I switch to the VS Code editor? Yes — enable Monaco in settings; your preference is remembered.