Creating middleware in NestJS - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the MiddlewareConsumer from NestJS.

NestJS

import { [1] } from '@nestjs/common';

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AModule

BController

CMiddlewareConsumer

DInjectable

Attempts:

3 left

💡 Hint

Common Mistakes

Importing Controller instead of MiddlewareConsumer.

Forgetting to import MiddlewareConsumer.

The MiddlewareConsumer is imported from @nestjs/common to configure middleware in NestJS.

2fill in blank

medium

Complete the code to implement the NestJS middleware interface.

NestJS

export class LoggerMiddleware implements [1] {

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ANestMiddleware

BPipeTransform

CExceptionFilter

DCanActivate

Attempts:

3 left

💡 Hint

Common Mistakes

Using PipeTransform instead of NestMiddleware.

Not implementing any interface.

Middleware classes in NestJS implement the NestMiddleware interface.

3fill in blank

hard

Fix the error in the middleware method signature.

NestJS

use(req: Request, res: Response, [1]) {

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Anext: () => void

Bnext: NextFunction

Cnext: Function

Dnext: any

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect type like () => void or any.

Omitting the next parameter.

The use method in NestJS middleware receives next as a NextFunction type from Express.

4fill in blank

hard

Fill both blanks to apply middleware to all routes in a module.

NestJS

configure(consumer: [1]) {
  consumer
    .apply(LoggerMiddleware)
    .[2]('*');
}

Drag options to blanks, or click blank then click option'

AMiddlewareConsumer

BModuleRef

CforRoutes

Duse

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong parameter type in configure method.

Using 'use' instead of 'forRoutes' to apply middleware.

The configure method receives a MiddlewareConsumer to apply middleware. The forRoutes method applies it to all routes.

5fill in blank

hard

Fill all three blanks to create a middleware that logs the request method and URL, then calls next.

NestJS

export class LoggerMiddleware implements NestMiddleware {
  use(req: [1], res: [2], [3]) {
    console.log(`${req.method} ${req.url}`);
    next();
  }
}

Drag options to blanks, or click blank then click option'

ARequest

BResponse

Cnext: NextFunction

DHttpException

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect types for req or res.

Omitting the next parameter or using wrong type.

The middleware use method parameters are Request, Response, and next of type NextFunction. This allows logging and passing control to the next middleware.

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