Express.js Custom & Third-Party Middleware Interview Questions

Q1. How do you create custom middleware in Express?

Custom middleware is simply a function with (req, res, next) signature.
Example:

const logger = (req, res, next) => {
  console.log(`${req.method} ${req.url}`);
  next();
};
app.use(logger);

It can perform any logic before passing control.

Q2. What is the CORS middleware and why use it?

CORS (Cross-Origin Resource Sharing) is a security mechanism.
The cors package adds CORS headers to responses, allowing your API to be accessed from different domains.
Example: const cors = require('cors'); app.use(cors());
Without it, browsers block cross-origin requests.

Q3. What was body-parser and why was it used?

body-parser was a popular third-party middleware to parse request bodies.
Before Express 4.16, it was needed for JSON and URL-encoded data.
Now, its functionality is built into Express (express.json(), express.urlencoded()), so you rarely need the separate package.

Q4. How do you create middleware with configuration options?

Create a function that accepts options and returns the middleware.
Example:

const requestLogger = (format) => (req, res, next) => {
  console.log(format, req.method, req.url);
  next();
};
app.use(requestLogger('Request:'));

This pattern is common in third-party middleware.

Q5. What are some commonly used third-party middleware?

Popular ones include:
• morgan (logging)
• helmet (security headers)
• cors (CORS support)
• compression (gzip compression)
• express-session (session management)
• multer (file uploads)
• passport (authentication)

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