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Role of libuv in the Node.js runtime
Libuv provides the event loop, a thread pool for blocking work, and OS async I/O abstractions.
Microtask versus macrotask execution order in Node
NextTick drains before promises, both microtask queues flush fully between each macrotask, timers and setImmediate are macrotasks.
Detecting and diagnosing event loop lag
Measure delay between scheduled and actual timer fire, expose it as a metric, find synchronous CPU-bound code.
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.
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.
Purpose of an ORM like Sequelize
Maps rows to objects, gives a model-based API, handles associations, migrations, and parameterized queries across dialects.
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.
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.
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.
Callback hell and how to refactor it
Deeply nested callbacks (pyramid of doom) hurt readability and error handling, refactor with promises or async/await.
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.
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.
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.
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.
Diagnosing intermittent crashes with PM2
PM2 auto-restarts and runs cluster mode for availability, inspect logs and metrics, watch memory for leaks, capture errors.
Explain the difference between IaaS, PaaS, and SaaS with examples
This tests your grasp of cloud abstraction layers and shared responsibility. A strong answer maps IaaS to raw infrastructure like EC2, PaaS to managed runtimes like Heroku, and SaaS to end-user apps like Gmail.
How does shared responsibility shift between IaaS and SaaS?
Tests your understanding of security ownership across cloud stacks. Strong answer: in IaaS you own OS, apps, and network controls; in SaaS you only own data, identities, endpoints, and accounts while the provider manages the rest.
CapEx vs OpEx in cloud migration
CapEx is large upfront asset spend, OpEx is ongoing pay-as-you-go cost; cloud shifts spending from CapEx to OpEx, trading ownership for flexibility.
IaaS vs PaaS for first cloud migration
IaaS (lift-and-shift) gives control with high ops overhead; PaaS lowers ops but may need refactoring; for a monolith with limited expertise, IaaS lift-and-shift is the lower-risk first step.
Scalability vs elasticity in the cloud
Scalability is the ability to handle more load by adding capacity; elasticity is automatically adding AND removing capacity in real time to match demand.