Intermediate interview questions in Node.js & Express
Order of the Node.js event loop phases
Timers, pending callbacks, poll, check, close phases in order; I/O completion runs in poll.
nextTick vs setImmediate vs setTimeout(fn, 0)
NextTick is a microtask that drains before the loop continues; setImmediate runs in check; setTimeout(0) in timers.
Offloading CPU-bound work with Worker Threads
Synchronous CPU work freezes the loop and all requests; offload to a Worker, communicate via messages or SharedArrayBuffer, use a pool.
Why package-lock.json must be committed
Lockfile pins exact versions of the whole dependency tree including transitive deps; guarantees identical installs across machines and CI.
Choosing between CommonJS and ES Modules
ESM is the standard with static import/export and top-level await; set type to module; CJS uses require and module.exports and is synchronous.
Layered structure for a scalable Express API
Routes map URLs, controllers handle HTTP, services hold business logic, data layer talks to the DB; keep each layer ignorant of HTTP except controllers.
SemVer and the caret vs tilde range operators
MAJOR.MINOR.PATCH signals breaking/feature/fix; caret allows minor and patch updates, tilde allows only patch; use tilde for tighter control.
Output order of sync, microtask, and macrotask
C runs first synchronously, then B drains from the microtask queue, then A from the macrotask queue.
Running independent requests with Promise.all and race
Start all requests then await Promise.all to get all results or fail fast on first rejection; use Promise.race when only the fastest settled result matters.
Handling async errors in Express middleware
Await inside try/catch and call next(err) on failure, or wrap the handler in an async error adapter; never let a rejected Promise go uncaught.
Comparing the three async error-handling styles
Callbacks pass err as first arg; Promises route errors to catch; async/await uses try/catch; an unhandled rejection can crash the Node process.
Counting lines in a 5GB log file efficiently
ReadFile loads all 5GB into RAM and may exceed buffer limits, instead stream with createReadStream plus readline and count line by line.
Reading a POST body from the request stream
Body arrives in chunks via data events, accumulate them, on the end event concatenate and JSON.parse inside try/catch.
path.resolve vs path.join
Join concatenates and normalizes relative segments, resolve builds an absolute path right to left from cwd and resets on any absolute segment.
fs.watch vs fs.watchFile
Watch uses OS event notifications, efficient but inconsistent across platforms, watchFile polls stat at an interval, reliable but slower.
Parsing JSON bodies with express.json
Register express.json via app.use so it reads the request stream and populates req.body before handlers run, place it before routes.
res.send vs res.json vs res.end
Send is flexible and sets content type by type, json serializes and sets JSON content type, end is the raw http terminator with no body helpers.
Express route order and matching
Express checks routes in definition order, /items/new matches the literal route first, if /:id came first new would be captured as an id.
Application-level vs router-level middleware
App.use() binds to the app and runs everywhere; router.use() binds to a Router and runs only for that router's routes.
Express error-handling middleware signature
(err, req, res, next) with err first, identified by arity, placed last after all routes.
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