Functional middleware in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this NestJS functional middleware?

Consider this functional middleware that logs request method and URL. What will be logged when a GET request is made to '/api/data'?

NestJS

import { Request, Response, NextFunction } from 'express';

export function logger(req: Request, res: Response, next: NextFunction) {
  console.log(`${req.method} ${req.url}`);
  next();
}

A Logs nothing and immediately sends a 404 response

B Logs: 'POST /api/data' and stops request processing

C Throws a runtime error because next() is missing

D Logs: 'GET /api/data' and passes control to next middleware

💡 Hint

Remember that next() passes control to the next middleware or route handler.

The middleware logs the HTTP method and URL, then calls next() to continue processing. Since the request is GET to '/api/data', it logs 'GET /api/data'.

📝 Syntax

intermediate

Which option correctly defines a functional middleware in NestJS?

Select the option that correctly defines a functional middleware that adds a custom header 'X-Custom' with value 'NestJS' to the response.

A export const customHeader = (req, res, next) => { res.headers['X-Custom'] = 'NestJS'; next(); }

B export function customHeader(req, res, next) { res.setHeader('X-Custom', 'NestJS'); next(); }

C export function customHeader(req: Request, res: Response, next: NextFunction) { res.setHeader('X-Custom', 'NestJS'); next(); }

D export function customHeader(req: Request, res: Response) { res.setHeader('X-Custom', 'NestJS'); }

💡 Hint

Check for correct TypeScript types and calling next() to continue.

Option C correctly types the parameters and calls next() after setting the header. Option C misses types, C uses incorrect header syntax, D misses next parameter and call.

❓ lifecycle

advanced

When is functional middleware executed in NestJS request lifecycle?

Choose the correct statement about when functional middleware runs during a request in NestJS.

A Before any route handler or controller method is called

B After the controller method but before the response is sent

C Only after all guards and interceptors have run

D Only after the response is sent to the client

💡 Hint

Middleware runs early to modify request or response before controllers.

Functional middleware runs before route handlers and controllers, allowing it to modify request or response early in the lifecycle.

🔧 Debug

advanced

Why does this functional middleware cause the request to hang?

Examine this middleware and select the reason the request never completes.

NestJS

export function hangMiddleware(req, res, next) {
  console.log('Middleware started');
  // Missing next() call
}

A Because res.send() is called twice causing an error

B Because next() is never called, the request never moves forward

C Because the middleware throws an exception before next()

D Because the middleware modifies req.url incorrectly

💡 Hint

Middleware must call next() to continue the request chain.

Without calling next(), the middleware blocks the request from continuing, causing it to hang.

❓ state_output

expert

What is the value of req.customData after this functional middleware runs?

Given this middleware that adds a customData property to the request, what will be the value of req.customData in the next middleware?

NestJS

import { Request, Response, NextFunction } from 'express';

interface CustomRequest extends Request {
  customData?: string;
}

export function addCustomData(req: CustomRequest, res: Response, next: NextFunction) {
  req.customData = 'NestJS Rocks!';
  next();
}

A 'NestJS Rocks!'

B null

C undefined

D Throws a TypeError because customData is not defined on Request

💡 Hint

The middleware adds customData property before calling next().

The middleware sets req.customData to 'NestJS Rocks!' so the next middleware sees this value.