Express.js Quiz ( Intermediate ) - All Questions

This intermediate-level Express.js quiz is designed for developers who already know the basics and want to test applied, real-world concepts. It focuses on middleware flow, routing patterns, error handling, REST APIs, and Express internals commonly asked in interviews.

Question 1: What is the main purpose of Express middleware?

Render UI
Handle database queries
Process requests before sending a response
Compile JavaScript

Explanation: Middleware functions execute during the request-response lifecycle.

Question 2: In which order does Express execute middleware?

Alphabetical order
Random order
Order of definition
Based on priority flags

Explanation: Middleware executes in the order it is defined.

Question 3: What happens if next() is not called in middleware?

Server crashes
Request hangs
Response auto-sends
Next route runs

Explanation: Without next(), the request lifecycle stops.

Question 4: Which middleware signature is used for error handling?

(req, res)
(req, res, next)
(err, req, res, next)
(error, res)

Explanation: Error middleware has four parameters.

Question 5: Where should error-handling middleware be placed?

At the top
After routes
Before app.listen()
Inside routers only

Explanation: Error middleware must be defined after routes.

Question 6: What does app.use('/api', router) do?

Creates API automatically
Mounts router at /api path
Limits HTTP methods
Adds middleware globally

Explanation: It mounts all router paths under /api.

Question 7: Which method is best for updating an entire resource?

POST
PATCH
PUT
GET

Explanation: PUT replaces the entire resource.

Question 8: Which method is best for partial updates?

POST
PUT
PATCH
DELETE

Explanation: PATCH updates part of a resource.

Question 9: What does req.headers contain?

Query parameters
Request body
HTTP headers
Route params

Explanation: req.headers stores HTTP header information.

Question 10: How can you send a custom status code with JSON?

res.json(400)
res.send(400)
res.status(400).json()
res.code(400).send()

Explanation: res.status().json() sends status with JSON.

Question 11: What is the purpose of express.Router()?

Start server
Create modular route handlers
Parse JSON
Handle errors

Explanation: Routers help organize routes into modules.

Question 12: What does req.params return?

Query strings
POST data
Route parameters
Headers

Explanation: req.params contains route parameters.

Question 13: What is the difference between req.params and req.query?

Both are same
params from URL path, query from URL string
query from body, params from headers
params only for POST

Explanation: params come from route path, query from URL string.

Question 14: Which middleware enables Cross-Origin Resource Sharing?

helmet
cors
morgan
dotenv

Explanation: cors enables CORS in Express.

Question 15: Why is helmet commonly used?

Performance optimization
Logging
Securing HTTP headers
Routing

Explanation: Helmet improves security by setting headers.

Question 16: Which status code indicates successful creation?

Explanation: 201 indicates a resource was created.

Question 17: Which status code indicates no content?

Explanation: 204 means success with no response body.

Question 18: How do you handle async errors properly in Express?

try-catch only
Callbacks only
Pass error to next()
Ignore errors

Explanation: Passing errors to next() triggers error middleware.

Question 19: Which pattern is best for structuring Express apps?

Single file
MVC pattern
Inline handlers
Global variables

Explanation: MVC improves maintainability.

Question 20: What does express.json() replace?

fs module
body-parser json middleware
cors
helmet

Explanation: express.json() replaces body-parser for JSON.

Question 21: Which HTTP method should be idempotent?

POST
PUT
PATCH
CONNECT

Explanation: PUT is idempotent by design.

Question 22: What is the role of next(err)?

Stops server
Skips remaining middleware
Triggers error handler
Restarts request

Explanation: Passing err invokes error middleware.

Question 23: Which Express method handles all HTTP verbs for a route?

app.any()
app.use()
app.all()
app.route()

Explanation: app.all() handles all HTTP methods.

Question 24: What does res.sendStatus(404) do?

Sends JSON
Sends status only
Redirects request
Throws error

Explanation: sendStatus sets status and sends its message.

Question 25: Why should validation middleware run early?

Better UI
Fail fast on bad requests
Faster routing
Improved SEO

Explanation: Early validation prevents unnecessary processing.

Question 26: Which object stores environment variables?

req.env
process.env
app.env
config.env

Explanation: process.env stores environment variables.

Question 27: What does app.param() handle?

Query params
Route parameters preprocessing
Body parsing
Headers

Explanation: app.param runs logic for route parameters.

Question 28: Which status code indicates a client error?

Explanation: 4xx codes represent client errors.

Question 29: What happens if two routes match a request?

Last route runs
All routes run
First matching route runs
Server crashes

Explanation: Express executes the first matching route.

Question 30: Which Express feature supports REST API versioning?

Query params
URL prefixing
Headers only
Cookies

Explanation: Versioning is commonly done using URL prefixes.

Question 31: Why should you avoid heavy logic in route handlers?

SEO issues
Poor readability and scalability
Routing errors
Security flaws

Explanation: Heavy logic should be moved to services.

Question 32: Which middleware parses form-data?

express.json()
multer
cors
helmet

Explanation: Multer handles multipart/form-data.

Question 33: What does res.set() do?

Sets response body
Sets HTTP headers
Sets route params
Sets cookies

Explanation: res.set() sets response headers.

Question 34: Which response method ends the response?

res.send()
res.end()
res.json()
res.redirect()

Explanation: res.end() ends the response.

Question 35: What is the default content type of res.json()?

text/plain
text/html
application/xml
application/json

Explanation: res.json() sets application/json.

Question 36: Why should Express apps be stateless?

Simpler routing
Better horizontal scaling
Faster rendering
Cleaner syntax

Explanation: Stateless apps scale more easily.

Question 37: Which method is used to serve static assets?

app.assets()
app.public()
express.static()
res.static()

Explanation: express.static serves static files.

Question 38: What does req.originalUrl provide?

Base URL only
Modified URL
Full original request URL
Query string only

Explanation: originalUrl contains the full request path.

Question 39: Which Express feature improves maintainability?

Inline logic
Router separation
Global state
Hardcoded values

Explanation: Separating routers improves maintainability.

Question 40: Which middleware should handle authentication?

After response
Before protected routes
Inside controllers only
In database layer

Explanation: Auth middleware must run before protected routes.

Question 41: Which HTTP status code indicates conflict?

Explanation: 409 indicates a conflict.

Question 42: What does res.redirect(301) mean?

Temporary redirect
Permanent redirect
Unauthorized
Server error

Explanation: 301 indicates permanent redirect.

Question 43: Which Express pattern avoids callback nesting?

Nested middleware
Async/await
Global handlers
Inline callbacks

Explanation: Async/await improves readability.

Question 44: Which layer should handle business logic?

Routes
Controllers or services
Middleware only
Views

Explanation: Business logic belongs in controllers or services.

Question 45: What is the primary goal of Express.js?

Frontend rendering
Minimal and flexible backend framework
Database management
Mobile app development

Explanation: Express is minimal and flexible for backend APIs.

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Express.js Quiz ( Intermediate )

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