Top 30 Intermediate Node.js & Express Interview Questions and Answers

Question 1

In which event loop phase do the majority of completed I/O callbacks, such as a finished network read, actually execute?

Accepted Answer

The poll phase, which retrieves and runs I/O completions

Answer

The check phase, alongside setImmediate callbacks

Answer

The close callbacks phase, after the loop drains

Answer

The timers phase, right after expired setTimeout callbacks

Question 2

When both are scheduled from inside a completed fs I/O callback, which runs first and why?

Accepted Answer

setImmediate, because the check phase follows poll in the same iteration

Answer

setTimeout(fn, 0), because timers always precede check

Answer

They run simultaneously since both mean zero delay

Answer

process.nextTick, because it is also a macrotask

Question 3

Why does wrapping a heavy synchronous computation in an async function fail to keep a Node server responsive?

Accepted Answer

The computation is still synchronous and never yields the single JS thread

Answer

Promises lower the priority of CPU work in the loop

Answer

Async functions run on a separate OS thread automatically

Answer

The garbage collector preempts long Promises

Question 4

What core problem does committing package-lock.json solve that package.json alone cannot?

Accepted Answer

It guarantees every install resolves to the exact same dependency tree, including transitive packages

Answer

It makes installs faster by caching tarballs in the repo

Answer

It documents which packages are devDependencies

Answer

It prevents anyone from adding new dependencies

Question 5

Which statement about using ES Modules instead of CommonJS in Node is accurate?

Accepted Answer

ESM lacks __dirname by default and is enabled via type module or .mjs

Answer

CommonJS supports top-level await but ESM does not

Answer

You can require an ESM-only file directly from a CommonJS module

Answer

import is just an alias for require with identical semantics

Question 6

Why should the service layer in a layered Express API avoid referencing the request and response objects?

Accepted Answer

It keeps business logic framework-agnostic and unit-testable without HTTP

Answer

Services run in a different thread from controllers

Answer

Express forbids accessing them outside route handlers

Answer

Passing them is slower than passing plain values

Question 7

Given a dependency written as ^1.4.2, which upgrade would npm refuse to install on its own?

Accepted Answer

2.0.0, a major release

Answer

1.5.3, a later minor and patch

Answer

1.4.9, a patch release

Answer

1.7.0, a minor release

Question 8

Why does the Promise callback print before the setTimeout(0) callback despite both being scheduled in the same tick?

Accepted Answer

The microtask queue is fully drained before the next macrotask runs

Answer

The order is implementation-defined and can vary

Answer

Promises run on a higher-priority OS thread

Answer

setTimeout has an enforced minimum delay larger than the Promise

Question 9

What happens to the remaining requests when one input to Promise.all rejects?

Accepted Answer

The combined Promise rejects immediately, but the other requests still run to completion

Answer

Promise.all waits for all of them before reporting the rejection

Answer

Their results replace the rejected one

Answer

They are automatically cancelled by the runtime

Question 10

In classic Express 4, how should an error from an awaited database call inside async middleware reach the error handler?

Accepted Answer

Catch it and pass it to next(err) so the error-handling middleware runs

Answer

Throw it and let Express catch it automatically

Answer

Ignore it, since unhandled rejections are routed by default

Answer

Call res.send with the error directly from the middleware

Question 11

Why might a try/catch fail to catch an error from an async call inside it?

Accepted Answer

If the returned Promise is not awaited, control leaves the block before it rejects

Answer

try/catch never works with Promises under any circumstance

Answer

catch only catches synchronous TypeErrors

Answer

async functions disable the surrounding try/catch

Question 12

Why does fs.createReadStream with readline scale to a 5GB log while fs.readFile does not?

Accepted Answer

Streaming keeps memory roughly constant by processing small chunks instead of buffering all 5GB

Answer

readline compresses the file before reading it

Answer

readFile cannot open files larger than 2GB on any platform

Answer

createReadStream reads the file twice as fast on disk

Question 13

Why must you wait for the request's end event before calling JSON.parse on a POST body?

Accepted Answer

The body arrives as multiple chunks and is only complete once end fires

Answer

The end event decrypts the body which is otherwise unreadable

Answer

req.body is populated synchronously but only readable after end

Answer

JSON.parse only works inside event handlers named end

Question 14

Given path.resolve('/foo', 'bar', '/baz', 'qux'), what is returned and why?

Accepted Answer

/baz/qux because an absolute segment discards everything to its left

Answer

qux because resolve uses only the last segment

Answer

/foo/bar/baz/qux because resolve concatenates like join

Answer

/foo/bar/qux because /baz is treated as a filename

Question 15

What is the core mechanism difference between fs.watch and fs.watchFile?

Accepted Answer

fs.watch uses OS-native change notifications; fs.watchFile polls with stat on an interval

Answer

Both use OS events, but fs.watchFile adds encryption

Answer

Both poll, but fs.watch polls faster

Answer

fs.watch polls stat on an interval; fs.watchFile uses OS events

Question 16

Where must app.use(express.json()) be registered for req.body to be available in a POST handler?

Accepted Answer

Before the route handlers, so the body is parsed when they run

Answer

Inside each route handler that needs the body

Answer

Position does not matter because Express reorders middleware

Answer

After the route handlers, so it can clean up the body

Question 17

When returning data from a JSON API, why is res.json preferred over res.end for sending an object?

Accepted Answer

res.json serializes the object and sets application/json, while res.end does neither

Answer

res.end is deprecated and will be removed from Express

Answer

res.json is faster because it skips serialization

Answer

res.end cannot finalize a response without a status code

Question 18

If app.get('/items/:id') is defined before app.get('/items/new'), what happens for a request to /items/new?

Accepted Answer

The /items/:id route matches first with id equal to 'new', shadowing the literal route

Answer

Both handlers run in sequence

Answer

Express returns a 404 due to the ambiguity

Answer

Express picks the literal /items/new because it is more specific

Question 19

You want authentication to apply only to routes under /admin without touching public routes. Which approach scopes it correctly?

Accepted Answer

router.use(requireAuth) on the admin Router, mounted at /admin

Answer

app.use(requireAuth) at the top of the main app file

Answer

Put requireAuth inside express.json()

Answer

Add requireAuth to every route via res.use()

Question 20

You wrote an error handler as (req, res, next) and errors passed via next(err) never reach it. What is the fix?

Accepted Answer

Declare four parameters: (err, req, res, next)

Answer

Wrap it in express.json()

Answer

Move the handler to the very top of the app

Answer

Rename next to done

Question 21

Auth middleware sets the current user so the route handler can use it. What is the correct, concurrency-safe place to store that user?

Accepted Answer

On the req object, e.g. req.user

Answer

On the res object's status code

Answer

In a module-level variable shared across requests

Answer

In a global process.env entry

Question 22

You mount a products router with app.use('/products', router) and inside it write router.get('/products/:id', ...). Requests to /products/42 return 404. Why?

Accepted Answer

The path becomes /products/products/:id because the mount prefix is added again

Answer

router.get only matches POST requests

Answer

express.Router cannot take path parameters

Answer

The router must be mounted with app.get not app.use

Question 23

A mobile client retries a request after a network timeout. Which request can it safely retry without risking duplicate resources?

Accepted Answer

PUT /users/42 with the full updated representation

Answer

Any request, since HTTP retries are always safe

Answer

POST /orders to place an order

Answer

POST /users with a new user payload

Question 24

GET /products/999 finds no such product, and separately a query fails because the database is unreachable. What status codes fit each case?

Accepted Answer

404 for the missing product, 500 (or 503) for the database failure

Answer

400 for the missing product, 404 for the database failure

Answer

404 for both, since the data was not returned

Answer

500 for both, since both are errors

Question 25

BlogPost.findById(id).populate('author') returns author as a raw ObjectId instead of the full document. What is the most likely cause?

Accepted Answer

The author field in the schema is missing the ref to the Author model

Answer

MongoDB performed a join that failed

Answer

populate only works on arrays, not single references

Answer

The query needs .lean() to populate

Question 26

Why are committed Sequelize migrations preferred over each developer manually altering their local database?

Accepted Answer

They give every environment the same ordered, reproducible, reversible schema changes

Answer

They eliminate the need for a database connection

Answer

They make queries run faster at runtime

Answer

They automatically generate seed data for tests

Question 27

A pre('save') hook hashes the password every time without checking isModified('password'). What goes wrong when a user later updates only their email?

Accepted Answer

The already-hashed password is hashed again, so login stops working

Answer

The password field is dropped from the document

Answer

Mongoose throws because the hook lacks next()

Answer

The email update is silently rejected

Question 28

Across five stateless Express instances behind a load balancer, what is the main operational cost of choosing session-based auth over JWTs?

Accepted Answer

You need a shared session store so any instance can resolve the session

Answer

Sessions require asymmetric keys on every node

Answer

Sessions cannot work with more than one server

Answer

Sessions expose the user id in the cookie

Question 29

In a Passport local strategy verify callback, what is the correct way to signal a successful authentication?

Accepted Answer

return done(null, user)

Answer

return done(null, true)

Answer

return done(user)

Answer

return done(true, user)

Question 30

In an Express 4 app, an async route handler throws an error but the centralized error middleware never runs. Why?

Accepted Answer

Express 4 does not auto-catch rejected promises, so the error must be forwarded via next(err)

Answer

Error middleware cannot handle async routes at all

Answer

The handler must return a string for errors to propagate

Answer

Async handlers require the GET method