Applying middleware to routes in NestJS - Cheat Sheet & Quick Revision

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, or end the request-response cycle.

beginner

How do you apply middleware to a specific route in NestJS?

You apply middleware by implementing the configure() method in a module and using consumer.apply(MiddlewareClass).forRoutes('route').

beginner

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

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

intermediate

Can middleware be applied globally in NestJS? How?

Yes, middleware can be applied globally by using app.use() in the main bootstrap file before starting the app.

intermediate

What types of routes can middleware be applied to in NestJS?

Middleware can be applied to string paths, controller classes, or route objects specifying method and path.

In NestJS, where do you define middleware for specific routes?

A Inside the module's configure() method using consumer.apply()

B Inside the controller's constructor

C In the main.ts file only

D Inside the service class

Middleware for specific routes is defined in the module's configure() method using consumer.apply().

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

A The request will automatically continue

B The request will hang and not reach the route handler

C An error is thrown immediately

D The middleware is skipped

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

Which method applies middleware globally in NestJS?

A controller.use()

B consumer.apply()

C app.use()

D module.use()

app.use() applies middleware globally before any route handlers.

Middleware in NestJS can be applied to which of the following?

A String paths, controller classes, route objects

B Only string paths

C Only controller classes

D Only HTTP methods

Middleware can be applied flexibly to string paths, controller classes, or route objects specifying method and path.

What is the main role of middleware in NestJS?

A To replace controllers

B To define routes

C To handle database connections

D To process requests before they reach route handlers

Middleware processes requests before they reach route handlers, often for logging, authentication, or modifying requests.

Explain how to apply middleware to a specific route in NestJS and why you might want to do this.

Think about controlling which routes get extra processing.

Describe the difference between applying middleware globally and applying it to specific routes in NestJS.

Consider the scope of middleware application.

Practice

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

easy

A. To run code before the route handler processes the request

B. To directly send responses without reaching the route handler

C. To replace the route handler with a new function

D. To automatically generate API documentation

Solution

1. Step 1: Understand middleware role

   Middleware runs before the route handler to process or modify requests.

2. Step 2: Identify middleware effect on routes

   It does not replace handlers or generate docs but can log or check requests first.

3. Final Answer:

   To run code before the route handler processes the request -> Option A

4. Quick Check:

   Middleware runs before route handlers [OK]

Hint: Middleware always runs before route handlers [OK]

Common Mistakes:

• Thinking middleware replaces route handlers
• Confusing middleware with response sending
• Assuming middleware auto-generates docs

2. Which of the following is the correct syntax to apply a middleware named LoggerMiddleware to all routes in a NestJS module?

easy

A. consumer.use(LoggerMiddleware).forRoutes('/');

B. consumer.apply(LoggerMiddleware).forRoutes('/');

C. consumer.apply(LoggerMiddleware).forRoutes('*');

D. consumer.apply(LoggerMiddleware).toRoutes('*');

Solution

1. Step 1: Recall NestJS middleware syntax

   The correct method to apply middleware is consumer.apply(...).forRoutes(...).

2. Step 2: Identify correct route pattern for all routes

   Using '/' applies middleware to all routes; '*' is invalid.

3. Final Answer:

   consumer.apply(LoggerMiddleware).forRoutes('/'); -> Option B

4. Quick Check:

   Use apply + forRoutes('/') for all routes [OK]

Hint: Use apply() with forRoutes('/') to cover all routes [OK]

Common Mistakes:

• Using use() instead of apply()
• Using toRoutes() which is invalid
• Using '*' instead of '/' for all routes

3. Given this middleware application code in a NestJS module:

consumer.apply(AuthMiddleware).forRoutes('users');

Which routes will have the AuthMiddleware applied?

medium

A. All routes including '/users' and '/users/:id'

B. Only routes with path exactly '/users'

C. All routes in the application

D. No routes, because the path is invalid

Solution

1. Step 1: Understand forRoutes string behavior

   Passing 'users' applies middleware to all routes starting with '/users', including parameters.

2. Step 2: Confirm middleware coverage

   This includes '/users', '/users/123', '/users/profile', etc., not the entire app.

3. Final Answer:

   All routes including '/users' and '/users/:id' -> Option A

4. Quick Check:

   forRoutes('users') covers prefix routes [OK]

Hint: forRoutes('path') applies middleware to that path and subpaths [OK]

Common Mistakes:

• Thinking only exact path matches get middleware
• Assuming middleware applies globally without '*'
• Believing invalid path disables middleware

4. Examine this NestJS middleware setup:

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

What is the issue with this code?

medium

A. Middleware class must be decorated with @Injectable

B. Middleware is applied globally without specifying routes

C. Missing route argument in forRoutes causes a runtime error

D. MiddlewareConsumer is not imported correctly

Solution

1. Step 1: Check forRoutes usage

   The method forRoutes requires at least one argument specifying routes; empty call is invalid.

2. Step 2: Identify error type

   Calling forRoutes() without arguments causes a runtime error because no routes are defined.

3. Final Answer:

   Missing route argument in forRoutes causes a runtime error -> Option C

4. Quick Check:

   forRoutes() needs routes argument [OK]

Hint: Always provide routes to forRoutes() [OK]

Common Mistakes:

• Leaving forRoutes empty
• Assuming middleware applies globally without routes
• Ignoring missing @Injectable on middleware

5. You want to apply LoggerMiddleware only to POST requests on the /orders route in NestJS. Which approach correctly applies middleware only to POST /orders requests?

hard

A. Apply middleware globally and skip non-POST requests inside middleware

B. Use consumer.apply(LoggerMiddleware).forRoutes('orders'); and check method inside middleware

C. Use consumer.apply(LoggerMiddleware).forRoutes('orders'); without method filtering

D. Use consumer.apply(LoggerMiddleware).forRoutes({ path: 'orders', method: RequestMethod.POST });

Solution

1. Step 1: Understand method filtering in middleware

   NestJS allows specifying method with an object: { path, method } in forRoutes.

2. Step 2: Identify correct syntax for POST-only middleware

   Using consumer.apply(...).forRoutes({ path: 'orders', method: RequestMethod.POST }) applies middleware only to POST /orders.

3. Final Answer:

   Use consumer.apply(LoggerMiddleware).forRoutes({ path: 'orders', method: RequestMethod.POST }); -> Option D

4. Quick Check:

   Use object with path and method in forRoutes for method filtering [OK]

Hint: Use forRoutes({path, method}) to target HTTP methods [OK]

Common Mistakes:

• Applying middleware to all methods without filtering
• Checking method inside middleware instead of in forRoutes
• Applying middleware globally and filtering inside middleware