Express Interview Questions and Answers 2025

Express.js is a fast, unopinionated, minimalist web framework for Node.js. It provides a robust set of features for building web applications and APIs, making it easier to handle HTTP requests, manage routes, and create middleware.

Middleware functions are functions that have access to the request object, response object, and the next middleware function. They can execute any code, modify request/response objects, end the request-response cycle, or call the next middleware. Common uses include authentication, logging, and error handling.

Routing refers to determining how an application responds to client requests to specific endpoints (URIs). Express provides methods to handle different HTTP methods like GET, POST, PUT, DELETE. Routes can be defined using app.route() or express.Router().

Express uses error-handling middleware functions that take four arguments (err, req, res, next). These functions are defined last, after other middleware and routes. They catch both synchronous and asynchronous errors, allowing centralized error handling.

Template engines enable dynamic content rendering in Express by allowing you to embed JavaScript code within HTML files. Popular template engines include EJS, Pug, and Handlebars. They're configured using app.set('view engine', '[engine_name]').

Request bodies can be parsed using middleware like express.json() for JSON data or express.urlencoded() for form data. These middleware functions add the parsed data to req.body, making it easily accessible in route handlers.

Express can serve static files (images, CSS, JavaScript) using express.static middleware. Files in the specified directory become publicly accessible. Multiple static directories can be set up using multiple express.static() calls.

app.get() handles specific HTTP GET requests to a route, while app.use() mounts middleware for all HTTP methods. app.use() is more general and executes for any HTTP method on matching paths.

Sessions in Express can be implemented using express-session middleware. It creates a session object in req.session, allowing you to store user-specific data across requests. Sessions can be stored in memory, databases, or other stores.

Route parameters are named URL segments used to capture values at specific positions in the URL. They're defined using colon syntax (/:paramName) and accessed via req.params object in route handlers.

Authentication in Express can be implemented using Passport.js (a middleware) or custom middleware. It involves verifying user credentials, creating sessions/JWT tokens, and protecting routes. Best practices include using bcrypt for password hashing and implementing proper session management.

RESTful APIs in Express follow principles like statelessness, resource-based URLs, and appropriate HTTP methods. They include proper status codes, versioning, error handling, and CORS support. Routes are organized by resource and actions (/users GET for list, POST for create, etc).

Express Generator is a command-line tool (express-generator) that creates an Express application skeleton. It sets up a project structure with views, routes, error handling, and basic middleware. Customizable through command-line options for template engines and CSS processors.

Security best practices include: using helmet middleware for HTTP headers, implementing rate limiting, validating input, using CORS properly, securing cookies, implementing CSP, avoiding sensitive data exposure, using HTTPS, and keeping dependencies updated.

Clustering in Express uses Node's cluster module to create worker processes. The master process manages workers, distributes incoming connections, and handles worker failures. This improves performance by utilizing multiple CPU cores and provides better reliability.

Performance optimization includes: implementing caching (Redis/Memcached), using compression middleware, optimizing database queries, implementing proper logging, using load balancing, enabling gzip compression, and implementing efficient error handling.

Middleware chaining is executing multiple middleware functions sequentially for a route. Each middleware can modify request/response objects and pass control using next(). Order matters, and the chain stops if next() isn't called or response is sent.

API versioning can be implemented through URL paths (/api/v1/), custom headers, query parameters, or content negotiation. It helps maintain backward compatibility while adding new features. Express routers can be used to organize different versions.

Error handling patterns include: try-catch blocks in async functions, custom error classes, centralized error handling middleware, different handlers for development/production, proper error logging, and graceful handling of different error types.

Testing Express apps involves unit tests for routes/controllers using libraries like Jest/Mocha, integration tests with Supertest, mocking dependencies, testing middleware, and end-to-end testing. Should include test coverage and continuous integration.

Process managers like PM2 handle application deployment, monitoring, and automatic restarts. They manage clustering, load balancing, log management, and zero-downtime deployments. PM2 specifically offers features like startup scripts, environment management, and real-time monitoring dashboard.

Event emitters in Express extend Node's EventEmitter class, enabling asynchronous, event-driven architecture. They allow custom events, error handling patterns, and communication between different parts of the application. Important for implementing observers, handling long operations, and managing application state.

Real-time features combine Express with Socket.io or WebSocket libraries. Implementation involves setting up WebSocket server alongside Express, handling connection events, managing rooms/namespaces, implementing heartbeat mechanisms, and ensuring scalability with Redis for multi-server setups.

Microservices with Express involve creating independent services, implementing service discovery, using API Gateway for routing/load balancing, handling inter-service communication, implementing circuit breakers, and managing distributed logging/monitoring. Common tools include Netflix OSS stack or Kong.

Large-scale file handling involves using streams, implementing chunked uploads, processing files asynchronously, integrating with cloud storage (S3/GCS), handling concurrent uploads, implementing progress tracking, and managing cleanup tasks. Libraries like Multer or Busboy are commonly used.

Database connection pooling involves managing connection lifecycle, implementing retry mechanisms, handling connection timeouts, monitoring pool metrics, and optimizing pool size. Tools like pg-pool for PostgreSQL or connection pools in Sequelize/Mongoose are used with proper error handling.

OAuth2 implementation involves setting up authorization routes, handling tokens, implementing refresh mechanisms, securing client secrets, managing scopes, handling state parameters, and implementing proper session management. Often uses Passport.js with specific strategies.

Rate limiting involves tracking request counts per IP/user, implementing sliding windows, using Redis for distributed systems, handling burst traffic, implementing retry-after headers, and providing proper client feedback. Libraries like express-rate-limit or custom middleware are used.

GraphQL implementation involves setting up Apollo Server, defining schema/resolvers, handling authentication/authorization, implementing data loaders for N+1 problems, managing subscriptions, and optimizing query performance. Includes proper error handling and schema stitching.

Circuit breakers prevent cascading failures by monitoring service health, implementing timeout patterns, managing fallback behaviors, tracking error thresholds, and implementing recovery strategies. Libraries like Hystrix or Opossum are commonly used with proper monitoring.