Advanced interview questions in Node.js & Express
Circular dependencies in CommonJS modules
When A requires B which requires A, the cache returns A's partial exports; fields defined later are undefined at that moment.
Diamond dependencies and nested node_modules
Npm hoists one version to the top and nests the conflicting version under the dependent package; both coexist on disk.
Monorepo workspaces vs private npm packages
Workspaces give atomic cross-service changes and instant local linking but couple release cadence; private packages give versioned isolation but add publish overhead and version drift.
Bounded concurrency for many async requests
Chunk the array and await Promise.all per chunk, or run a fixed worker pool pulling from a shared index; cap in-flight requests.
Promise.all vs Promise.allSettled
All rejects on the first failure; allSettled always fulfills with a status/value or reason per input. Use allSettled when partial success is acceptable.
Async iterators and for await...of for streaming
Async iterators yield values lazily over time; for await...of consumes them sequentially with backpressure, keeping memory bounded.
Scaling across cores with cluster and os
Os.cpus gives core count, the primary forks one worker per core, all workers share the listening port, the OS load-balances connections.
Backpressure in Node.js streams
Backpressure pauses the readable when the writable buffer fills, pipe and pipeline manage it automatically, pipeline also propagates errors and cleans up.
http.Agent and connection pooling
The agent pools and keeps sockets alive, avoiding repeated TCP and TLS handshakes, controlled by keepAlive and maxSockets.
Modularizing routes with express.Router
Create a Router instance in users.js, attach routes to it, export it, then mount it under a base path with app.use in the main file.
Custom API key auth middleware
Read the header from req, on missing or invalid send res.status(401) and return, on valid call next, mount before protected routes.
Centralized error-handling middleware
Error middleware takes four args err, req, res, next, is defined last, and runs when next(err) is called or sync errors throw.
Write a JWT authentication middleware
Read the header, strip Bearer, jwt.verify with the secret, set req.user and next(), else send 401.
Conditionally apply middleware by request property
Use express.text({ type: 'application/xml' }) or a guard wrapper that checks req.is() then calls the parser or next().
Centralized error handling in an Express API
A final four-arg error middleware, an asyncHandler wrapper to funnel promise rejections via next, a custom error class with statusCode, returning uniform JSON.
API versioning: URL vs header strategies
Version via URL path (/v1/), a custom or Accept header, or a query param; URL is visible and cache-friendly, headers keep URLs clean but are less discoverable.
Solving the N+1 query problem in Sequelize
Define N+1 as one parent query plus one per child, detect it via SQL logging, fix with eager loading using include.
Atomic order creation with Sequelize transactions
Wrap dependent writes in sequelize.transaction, pass the transaction to each query, let managed transactions auto-commit or roll back.
MongoDB aggregation pipeline for total sales
Explain the pipeline as ordered stages, use $group with $sum to total per productId, $match on the computed total, then $sort descending.
Role-based access control middleware in Express
Authenticate first to set req.user, then a parameterized role-check middleware that compares req.user.role and returns 403 if it fails, applied to protected routes.
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