Repository debugging

Real codebases. Real bugs. Failing tests.

Each lab is a small multi-file service with a bug ticket and a test suite, the same shape as the repository-debugging rounds used in modern online assessments. Find the root causes, fix them, turn every test green. Everything runs in your browser.

0 / 23 solved

Movie Watchlist API

Saves nothing, deletes nothing, and the rate limiter is never wired in.

Mongoose find vs findOnepopulatesliding-window rate limiting

45m7 files8 tests6 bugs

Auth Service: Register, Login, JWT

Passwords stored in plain text and nobody can log in.

this binding in hooksbcryptJWT verify/expiry

35m5 files9 tests5 bugs

Inventory Stock & Error Handling

The error handler never runs and stock goes negative.

error middleware arityCastError{ new: true }

30m4 files7 tests4 bugs

Order History Pagination

Page 1 is empty, totals are wrong, the oldest orders show first.

offset paginationquery string parsingcountDocuments

20m3 files5 tests5 bugs

Checkout Pricing Engine

Totals come back as strings and coupons give 1000% off.

reduce seedfloating point moneytype coercion

20m3 files8 tests5 bugs

Product Page Response Cache

Every product page shows the same product, and the cache never caches.

this bindingcache keysTTL

30m4 files5 tests4 bugs

Async/Await Pitfalls in a Task API

Responses go out before the database work finishes.

awaitforEach + asyncPromise.all

25m3 files7 tests3 bugs

Meeting Room Double-Booking

Two teams keep landing in the same room at the same time.

interval overlapdate handlingquery operators

25m3 files4 tests3 bugs

Admin Panel Access Control

Unauthenticated users crash it; logged-in customers become admins.

authn vs authzmiddleware order= vs ===

25m5 files6 tests3 bugs

URL Shortener Service

The second link ever created crashes the service.

unique indexesatomic $incHTTP redirects

20m3 files6 tests4 bugs

Express Wiring & Route Order

Every endpoint answers 404 and /me is treated as a user id.

registration orderbody parsingroute shadowing

15m3 files5 tests3 bugs

Product Search, Sorting & Injection

Searching 'c++' crashes, sorting is random, and sellers log in without a key.

regex escapingsort comparatorsNoSQL injection

25m5 files6 tests4 bugs

Payments: Idempotency & Double Charges

Mobile retries charge customers twice, and $19.99 becomes 1998.9999 cents.

idempotency keyscheck-then-act racespromise memoization

35m3 files5 tests3 bugs

Retry with Exponential Backoff

The retry helper gives up early, waits wrong, and swallows errors.

exponential backoffloop boundserror propagation

20m1 files7 tests4 bugs

Notification Fan-out

One bad device token kills the whole campaign; every greeting says 'Hi undefined'.

Promise.allSettledclosuresvar vs let

20m2 files4 tests2 bugs

LRU Cache Gone Wrong

It evicts the newest item and never actually remembers anything.

Map insertion orderLRU evictionfalsy values

20m1 files6 tests4 bugs

Payment Webhook Receiver

Every webhook is rejected, forged ones get through, and paid orders show as refunded.

webhook signaturesreplay protectionseconds vs ms

30m5 files5 tests4 bugs

Monthly Sales Report

March shows up as February, the last day of the month vanishes, and averages are tiny.

zero-based monthsinclusive date rangesUTC bucketing

25m4 files4 tests4 bugs

Python: Bank Ledger Integrity

Every account shares one history, and money vanishes in failed transfers.

class vs instance attributesmutable defaultsDecimal money

25m1 files7 tests4 bugs

Python: Token Bucket Rate Limiter

One noisy client throttles everyone, and limits never recover.

token bucketshared referencesfloor vs true division

25m1 files6 tests4 bugs

Python: Access-Log Analyzer

The on-call dashboard crashes on empty logs and miscounts errors.

generatorsstring vs int comparisonregex whitespace

25m1 files6 tests4 bugs

Python: Meeting Scheduler

Free-time finder offers slots that are actually busy.

merge intervalsboundariesmutation during iteration

20m1 files6 tests4 bugs

Python: TTL Cache Decorator

EUR prices come back in USD and None is never cached.

decoratorsfunctools.wrapscache keys

15m1 files6 tests3 bugs