Intermediate everything in Node.js & Express
PM2 cluster mode and multi-core utilization?
Cluster mode forks multiple worker processes using Node.js cluster module, distributes load, uses all CPU cores.
dependencies vs devDependencies in production?
Dependencies are needed at runtime, devDependencies only for development. npm install includes both, npm ci --only=production excludes dev.
What are key Dockerfile steps for Node.js Express apps?
Use lightweight base image, copy app, install deps, expose port, set NODE_ENV, run.
How to sync state across multiple user devices?
Attach userId to each socket, track active sockets per user, broadcast user state to all their sockets.
How to authenticate WebSocket connections using JWTs?
Client sends JWT on connection, server validates via middleware, socket is attached to user.
How does Socket.IO handle WebSocket failures with fallbacks?
Transports are communication protocols, WebSocket is primary, long-polling is fallback, client auto-detects and downgrades.
How do Socket.IO rooms organize client communication?
Rooms group sockets for targeted broadcasting, socket can join multiple rooms, emit to room broadcasts to all members.
Diagnosing intermittent crashes with PM2
PM2 auto-restarts and runs cluster mode for availability, inspect logs and metrics, watch memory for leaks, capture errors.
Unit testing Express auth middleware in isolation
Build fake req/res, use a spy/mock for next and res methods, assert next called on valid token and 401 sent on invalid.
JWT login and protected route flow in Express
Verify credentials, sign a JWT, client stores and sends it (Authorization header or httpOnly cookie), middleware verifies signature on protected routes.
Defining many-to-many relationships in Sequelize
Use belongsToMany through a join table, the through table holds the foreign keys, eager-load courses with include.
Nested routes versus query params for related resources
Nested routes express ownership and scope clearly, query params on the flat resource are flexible for filtering and combining.
Callback hell and how to refactor it
Deeply nested callbacks (pyramid of doom) hurt readability and error handling, refactor with promises or async/await.
Blocking versus non-blocking I/O in Node
Blocking calls halt the thread until done, non-blocking returns immediately and notifies via callback or promise, fs.readFileSync versus fs.readFile.
Managing user presence with reconnection grace periods
Rely on heartbeats, apply a grace period before marking offline, reconcile reconnects by user not socket id, use a shared store for multi-instance.
Pinpointing validation errors in nested request data
Use schema validation that reports a path, collect all errors not just the first, return a 400 with field paths and messages.
Purpose of an ORM like Sequelize
Maps rows to objects, gives a model-based API, handles associations, migrations, and parameterized queries across dialects.
Validating request bodies with Express middleware
Run validation middleware before the handler, check email format and password length, return 400 with errors on failure, call next on success.
V8 generational GC and event loop responsiveness
Young-generation scavenges are frequent but short, old-generation major GC is rarer but longer, stop-the-world pauses block the single JS thread.
Detecting and diagnosing event loop lag
Measure delay between scheduled and actual timer fire, expose it as a metric, find synchronous CPU-bound code.
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