Creating middleware in NestJS - Try It Yourself

Creating middleware

📖 Scenario: You are building a simple NestJS application that needs to log every request's method and URL. Middleware is a perfect place to do this because it runs before the route handlers.

🎯 Goal: Create a middleware in NestJS that logs the HTTP method and URL of each incoming request.

📋 What You'll Learn

Create a middleware class named LoggerMiddleware

Implement the use method inside LoggerMiddleware

Log the request method and URL inside the use method

Apply the middleware to all routes in the main application module

💡 Why This Matters

🌍 Real World

Middleware is used in real web applications to handle tasks like logging, authentication, and request validation before the main route logic runs.

💼 Career

Understanding middleware is essential for backend developers working with NestJS or similar frameworks, as it helps manage cross-cutting concerns cleanly.

1

Create the LoggerMiddleware class

Create a class called LoggerMiddleware that implements NestMiddleware from @nestjs/common. Inside it, define an empty use method that takes req, res, and next as parameters.

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) {
    // Your code here
  }
}

Hint

Remember to import NestMiddleware and the types Request, Response, and NextFunction from Express.

2

Add logging inside the use method

Inside the use method of LoggerMiddleware, add a console.log statement that logs the HTTP method and URL of the incoming request using req.method and req.url. Then call next() to continue the request cycle.

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) {
    // Add console.log to show method and url
    // Your code here
  }
}

Hint

Use a template string to include req.method and req.url inside the log message.

3

Apply the LoggerMiddleware in the AppModule

In the AppModule class, implement the configure method that takes a MiddlewareConsumer parameter. Use consumer.apply(LoggerMiddleware).forRoutes('*') to apply the middleware to all routes.

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(`Request... Method: ${req.method}, URL: ${req.url}`);
    next();
  }
}

import { Module, MiddlewareConsumer } from '@nestjs/common';

@Module({})
export class AppModule {
  configure(consumer: MiddlewareConsumer) {
    // Apply LoggerMiddleware to all routes
    // Your code here
  }
}

Hint

Use the configure method and MiddlewareConsumer to apply middleware globally.

4

Complete the module imports and exports

Add LoggerMiddleware to the providers array in the @Module decorator of AppModule to register it properly.

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(`Request... Method: ${req.method}, URL: ${req.url}`);
    next();
  }
}

import { Module, MiddlewareConsumer } from '@nestjs/common';

@Module({
  // Add providers array here
  // Your code here
})
export class AppModule {
  configure(consumer: MiddlewareConsumer) {
    consumer.apply(LoggerMiddleware).forRoutes('*');
  }
}

Hint

Register the middleware class in the providers array inside the @Module decorator.

Practice

1. What is the main purpose of middleware in a NestJS application?

easy

A. To handle server errors automatically

B. To define database models

C. To run code before the request reaches the route handler

D. To create UI components

Solution

1. Step 1: Understand middleware role

   Middleware is code that runs before the main route handler processes a request.

2. Step 2: Compare options

   Only To run code before the request reaches the route handler describes this behavior correctly; others describe unrelated tasks.

3. Final Answer:

   To run code before the request reaches the route handler -> Option C

4. Quick Check:

   Middleware runs before routes [OK]

Hint: Middleware always runs before route handlers [OK]

Common Mistakes:

• Confusing middleware with controllers
• Thinking middleware handles UI rendering
• Assuming middleware manages database models

2. Which of the following is the correct way to define a middleware class in NestJS?

easy

A. const MyMiddleware = (req, res, next) => { next(); }

B. class MyMiddleware { handle(req, res, next) { next(); } }

C. function MyMiddleware(req, res, next) { next(); }

D. class MyMiddleware { use(req, res, next) { next(); } }

Solution

1. Step 1: Recall NestJS middleware pattern

   Middleware must be a class with a method named use that takes req, res, next.

2. Step 2: Check options

   Only class MyMiddleware { use(req, res, next) { next(); } } defines a class with a use method correctly. class MyMiddleware { handle(req, res, next) { next(); } } uses handle, which is invalid. Options C and D define functions, not classes.

3. Final Answer:

   class MyMiddleware { use(req, res, next) { next(); } } -> Option D

4. Quick Check:

   Middleware class must have use() method [OK]

Hint: Middleware class must have a use() method [OK]

Common Mistakes:

• Using method name other than use
• Defining middleware as a function instead of class
• Forgetting to call next() inside use

3. Given this middleware class, what will be logged when a request to '/some-path' hits the route?

class LoggerMiddleware {
  use(req, res, next) {
    console.log('Request URL:', req.url);
    next();
  }
}

medium

A. Request URL: /some-path

B. Request URL: undefined

C. No output because next() is missing

D. Error: use method not found

Solution

1. Step 1: Analyze middleware behavior

   The middleware logs the request URL from req.url and then calls next() to continue.

2. Step 2: Predict output

   When a request to '/some-path' arrives, it logs 'Request URL: /some-path'. No errors occur because use is defined and next() is called.

3. Final Answer:

   Request URL: /some-path -> Option A

4. Quick Check:

   Middleware logs req.url correctly [OK]

Hint: Middleware logs req.url before calling next() [OK]

Common Mistakes:

• Forgetting to call next() causing no further processing
• Assuming req.url is undefined
• Confusing method name use with handle

4. What is wrong with this middleware code?

class LoggerMiddleware {
  use(req, res, next) {
    console.log('Request received');
  }
}

export class AppModule implements NestModule {
  configure(consumer: MiddlewareConsumer) {
    consumer.apply(LoggerMiddleware).forRoutes('users');
  }
}

medium

A. Missing call to next() inside LoggerMiddleware

B. MiddlewareConsumer must be imported from '@nestjs/common' not '@nestjs/core'

C. The configure method should return a value

D. No semicolon after forRoutes call

Solution

1. Step 1: Check middleware code

   The registration uses MiddlewareConsumer correctly, but examine the LoggerMiddleware class.

2. Step 2: Identify the error

   The use method does not call next(), causing the request to hang without reaching the route handler.

3. Final Answer:

   Missing call to next() inside LoggerMiddleware -> Option A

4. Quick Check:

   Middleware must call next() to continue [OK]

Hint: Always call next() inside middleware use() method [OK]

Common Mistakes:

• Forgetting next() call causing request hang
• Thinking configure must return a value
• Confusing import sources for MiddlewareConsumer

5. You want to apply a middleware only to POST requests on the '/login' route. How can you achieve this in NestJS?

hard

A. Use consumer.apply(MyMiddleware).forRoutes('/login') and check method inside middleware

B. Use consumer.apply(MyMiddleware).forRoutes({ path: '/login', method: RequestMethod.POST })

C. Apply middleware globally and filter POST requests inside middleware

D. NestJS does not support method-specific middleware

Solution

1. Step 1: Understand method-specific middleware application

   In NestJS, you can specify route and HTTP method using an object with path and method in forRoutes.

2. Step 2: Evaluate options

   Use consumer.apply(MyMiddleware).forRoutes({ path: '/login', method: RequestMethod.POST }) correctly uses { path: '/login', method: RequestMethod.POST }. Use consumer.apply(MyMiddleware).forRoutes('/login') and check method inside middleware applies middleware to all methods on '/login', requiring manual filtering. Apply middleware globally and filter POST requests inside middleware applies globally, which is less efficient. NestJS does not support method-specific middleware is false.

3. Final Answer:

   Use consumer.apply(MyMiddleware).forRoutes({ path: '/login', method: RequestMethod.POST }) -> Option B

4. Quick Check:

   Use forRoutes with path and method object [OK]

Hint: Use forRoutes with path and method object for specific HTTP verbs [OK]

Common Mistakes:

• Applying middleware to all methods unintentionally
• Not importing RequestMethod enum
• Assuming method-specific middleware is unsupported