Node.js & Backend JS debugging

Server-side JavaScript debugging — event-loop lag, stream backpressure, ESM/CommonJS interop, Express/Fastify middleware, and the runtime errors that only show up under load.

41 guides·32 playable labs

Playable labs

The Phantom Env VarWorks locally. Fails in CI after a config change.Config & Environment · Easy Wrong Port, Right ServiceHealth check passes, traffic hits the admin service instead of the public API.Network & Connectivity · Easy The Stale LockDeploy pipeline stuck because an old lock never expired.CI/CD & Pipelines · Easy AccessDenied at 3AMProduction job fails only after rotating credentials.Secrets & Auth · Medium The Silent TimeoutRequests hang for 30 seconds, then succeed on retry.Network & Connectivity · Medium Cache StampedeEvery morning at 9AM service slows to a crawl for 90 seconds.Caching & Performance · Medium The N+1 That Wasn'tDB load explodes, but the query count looks normal.Database · Medium The Missing HeaderAPI works in browser, fails in worker/client.HTTP/API · Easy The Flaky RetryRetry logic makes the outage worse.Reliability · Medium The Broken Health CheckDeployment rolls back even though app is alive.Deployment · Easy The Timezone TrapReport fails only around midnight UTC.Date/Time · Medium The Case-Sensitive ConfigWorks on macOS, fails in Linux CI.Config & Environment · Easy Pagination off by onePage 1 skips the first N items.HTTP/API · Easy CORS origin parsing bugRegex origin validation allows malicious domains.Security · Medium Cache key missing tenant idMulti-tenant cache key uses just user_id, causing cross-tenant leaks.Caching & Performance · Hard The stale confirmationA successful update is followed by an old value.Databases & Consistency · Medium The missing trace IDA downstream request loses its correlation header.Observability · Easy Half an importA rejected batch still leaves earlier rows committed.Data Integrity · Hard The log that ate the requestA debug log serializes an entire upload payload.Performance & Observability · Medium The Price That Refused to UpdateMerchants change a price. Half the storefront still shows yesterday's number.Caching & Performance · Easy The Invoice That Forgot QuantityCarts with quantity > 1 undercharge. Finance finds missing line totals.Data Integrity · Medium The Retry That Amplified OutagesWhen auth fails, clients hammer it harder. Traffic multiplies the outage.Reliability · Easy The Header That Vanished Mid-FlightAuthenticated upstream calls 401 even though the client set Authorization.Network & Connectivity · Medium The Query That Loaded the WorldOrders list endpoint returns every tenant's rows and times out under load.Database · Medium The Filter That Swallowed False?includeDeleted=false still returns soft-deleted rows.HTTP/API · Medium The Rate Limit That Counted EveryoneOne noisy tenant trips the rate limit for every customer on the node.Reliability · Medium Duplicate webhook processingEach request registers another event listener, so later webhooks are processed multiple times.Events & Concurrency · Medium HTTP method casing mishandled for non-standard verbsA Request constructor accepts a method option. The library normalizes the six standard HTTP methods (DELETE, GET, HEAD, OPTIONS, POST, PUT) to uppercase regardless of input casing, while preserving every other method string exactly as supplied. Currently, the casing rule applies too broadly, altering method names that should pass through untouched.API & Request Flow · Easy Repeated trailing slashes in base URLWhen a base URL like 'https://api.github.com///' is joined with a path like '/users', the resulting URL contains an extra slash between the host and the path instead of a single separator.API & Request Flow · Easy Zero-value entries-read argument omitted from XGROUP tokensA caller passes the ENTRIESREAD option set to 0 to XGROUP CREATE or XGROUP SETID. The argument builder should emit the ENTRIESREAD keyword and a '0' token. Instead, when the option value is exactly 0, the trailing ENTRIESREAD and '0' tokens are missing from the produced array.Data & Serialization · Medium App.render rejects null and undefined options valuesA caller invokes `app.render(viewName, options, callback)` and expects the callback to deliver the rendered template string even when `options` is `null` or `undefined`. The callback does not deliver the expected rendered output for those cases, while rendering with a normal options object continues to succeed.Config & Environment · Easy Idempotency key ignoredThe payment endpoint validates an idempotency key but drops it before the database call.API & Request Flow · Medium

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