Creating middleware in NestJS - Quick Revision & Summary

Recall & Review

beginner

What is middleware in NestJS?

Middleware in NestJS is a function that runs before the route handler. It can modify the request or response objects or end the request-response cycle.

beginner

How do you create a middleware class in NestJS?

Create a class that implements the <code>NestMiddleware</code> interface and define a <code>use(req, res, next)</code> method.

intermediate

How do you apply middleware to routes in NestJS?

Use the configure() method in a module that implements NestModule and call consumer.apply(MiddlewareClass).forRoutes('route').

beginner

What is the role of the next() function in NestJS middleware?

The next() function passes control to the next middleware or route handler. Without calling it, the request will not proceed.

intermediate

Can NestJS middleware be asynchronous? How?

Yes, middleware can be asynchronous by making the use() method async and using await inside it before calling next().

Which interface must a NestJS middleware class implement?

A MiddlewareHandler

B MiddlewareInterface

C NestMiddleware

D RouteMiddleware

NestJS middleware classes must implement the NestMiddleware interface to define the use() method.

Where do you apply middleware in a NestJS application?

A Inside a service class

B Directly inside a controller method

C In the main.ts bootstrap file only

D In the configure() method of a module implementing NestModule

Middleware is applied in the configure() method of a module that implements NestModule using the consumer object.

What happens if you forget to call next() in middleware?

A The request will hang and not reach the route handler

B The request will skip the middleware

C The server will crash

D The middleware will run twice

Without calling next(), the request does not proceed to the next middleware or route handler, causing it to hang.

Which of these is NOT a valid way to apply middleware in NestJS?

A Adding middleware directly in the controller decorator

B consumer.apply(MyMiddleware).exclude('dogs').forRoutes('cats')

C consumer.apply(MyMiddleware).forRoutes('cats')

D Applying middleware globally in main.ts

Middleware cannot be added directly in controller decorators; it must be applied via the module's configure() or globally.

Can middleware modify the response object before sending it?

A No, middleware can only read the request

B Yes, middleware can modify req and res objects

C No, only interceptors can modify responses

D Yes, but only if it returns a value

Middleware can modify both request and response objects before passing control along.

Explain how to create and apply middleware in a NestJS module.

Think about the class, interface, and where you connect middleware to routes.

Describe the role of the next() function in NestJS middleware and what happens if it is not called.

Consider how middleware controls the flow of a request.

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