Creating middleware in NestJS - Why You Should Know This

The Big Idea

What if you could fix a bug in one place and instantly improve your whole app's security?

The Scenario

Imagine you have to check user login status on every single route in your app by writing the same code again and again inside each route handler.

The Problem

Manually repeating checks everywhere makes your code messy, hard to update, and easy to forget important steps, leading to bugs and security holes.

The Solution

Middleware lets you write that check once and apply it automatically to many routes, keeping your code clean and consistent.

Before vs After

✗ Before

@Get('/profile')
getProfile(@Req() req, @Res() res) {
  if (!req.user) return res.status(401).send('Login required');
  /* handler code */
}
@Get('/settings')
getSettings(@Req() req, @Res() res) {
  if (!req.user) return res.status(401).send('Login required');
  /* handler code */
}

✓ After

app.use(authMiddleware);
@Get('/profile')
profileHandler() {
  /* handler code */
}
@Get('/settings')
settingsHandler() {
  /* handler code */
}

What It Enables

You can easily add, remove, or change shared logic for many routes in one place, making your app safer and easier to maintain.

Real Life Example

In a shopping app, middleware can check if a user is logged in before letting them add items to their cart or checkout.

Key Takeaways

Writing repeated code for shared tasks is slow and risky.

Middleware centralizes common logic for many routes.

This keeps your app clean, secure, and easy to update.

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