Creating middleware in NestJS - Practice Exercises

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this NestJS middleware logging example?

Consider this NestJS middleware that logs the request method and URL. What will be logged when a GET request is made to '/users'?

NestJS

import { Injectable, NestMiddleware } from '@nestjs/common';
import { Request, Response, NextFunction } from 'express';

@Injectable()
export class LoggerMiddleware implements NestMiddleware {
  use(req: Request, res: Response, next: NextFunction) {
    console.log(`${req.method} ${req.url}`);
    next();
  }
}

A POST /users

B GET /users

C Error: req.method is undefined

D GET undefined

💡 Hint

Look at how req.method and req.url are used inside the middleware.

The middleware logs the HTTP method and URL of the incoming request. For a GET request to '/users', it logs 'GET /users'.

❓ lifecycle

intermediate

When is NestJS middleware executed in the request lifecycle?

At what point in the request lifecycle does NestJS middleware run?

A Only after the response is sent to the client

B After the route handler returns a response

C Before the route handler is called

D Only during application startup

💡 Hint

Middleware typically processes requests before controllers handle them.

NestJS middleware runs before the route handler to process or modify the request or response.

📝 Syntax

advanced

Which option correctly applies middleware to a specific route in NestJS?

You want to apply a middleware only to the '/api/products' route. Which code snippet correctly does this in NestJS?

NestJS

import { MiddlewareConsumer, Module, NestModule } from '@nestjs/common';
import { LoggerMiddleware } from './logger.middleware';

@Module({})
export class AppModule implements NestModule {
  configure(consumer: MiddlewareConsumer) {
    // Choose the correct option below
  }
}

A consumer.apply(LoggerMiddleware).forRoutes('/api/products');

B consumer.use(LoggerMiddleware).forRoutes('/api/products');

C consumer.apply(LoggerMiddleware).toRoutes('/api/products');

D consumer.use(LoggerMiddleware).toRoutes('/api/products');

💡 Hint

Check the NestJS MiddlewareConsumer API for the correct method names.

The correct method to apply middleware is 'apply', and the method to specify routes is 'forRoutes'.

🔧 Debug

advanced

Why does this NestJS middleware not log anything?

This middleware is supposed to log request info but logs nothing. What is the cause?

NestJS

import { Injectable, NestMiddleware } from '@nestjs/common';
import { Request, Response, NextFunction } from 'express';

@Injectable()
export class SilentMiddleware implements NestMiddleware {
  use(req: Request, res: Response, next: NextFunction) {
    // Missing console.log here
    next();
  }
}

A The middleware does not call console.log, so nothing is logged

B The middleware forgot to call next(), so it hangs

C The middleware is not decorated with @Injectable()

D The middleware uses wrong parameter types

💡 Hint

Look inside the use method for logging statements.

The middleware function does not have any console.log or logging code, so it produces no output.

🧠 Conceptual

expert

What happens if NestJS middleware does not call next()?

In NestJS, what is the effect of middleware that does not call the next() function?

A The request will skip the middleware and go directly to the route handler

B The middleware will complete and send a default response

C The middleware will throw a runtime error automatically

D The request will hang and never reach the route handler

💡 Hint

Think about how middleware chains work in Express-based frameworks.

Middleware must call next() to pass control; otherwise, the request stops and hangs.