Applying middleware to routes in NestJS - Step-by-Step Execution

Concept Flow - Applying middleware to routes

Define Middleware Function

↓

Create Middleware Class

↓

Apply Middleware in Module

↓

Specify Routes or Route Patterns

↓

Incoming Request Matches Route?

No→Skip Middleware

Yes↓

Middleware Executes

↓

Pass Control to Next Handler

↓

Route Handler Executes

Middleware is defined, applied to specific routes, and runs on matching requests before the route handler.

Execution Sample

NestJS

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

@Injectable()
export class LoggerMiddleware implements NestMiddleware {
  use(req: any, res: any, next: () => void) {
    console.log('Request logged');
    next();
  }
}

// In module
@Module({})
export class AppModule implements NestModule {
  configure(consumer: MiddlewareConsumer) {
    consumer.apply(LoggerMiddleware).forRoutes('cats');
  }
}

This code defines a LoggerMiddleware that logs requests and applies it only to the '/cats' route.

Execution Table

Step	Incoming Request URL	Route Match	Middleware Action	Next Called	Route Handler Called
1	/cats	Yes	Logs 'Request logged'	Yes	Yes
2	/dogs	No	Middleware skipped	N/A	Yes
3	/cats/123	No	Middleware skipped	N/A	Yes
4	/birds	No	Middleware skipped	N/A	Yes
5	End of requests	N/A	N/A	N/A	N/A

💡 Requests not matching 'cats' route skip middleware; matching requests run middleware then route handler.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
req.url	/start	/cats	/dogs	/cats/123	/birds	End
middlewareExecuted	false	true	false	false	false	N/A
routeHandlerExecuted	false	true	true	true	true	N/A

Key Moments - 2 Insights

Why does the middleware not run for the '/dogs' request?

Because the middleware is applied only to routes matching 'cats', so '/dogs' does not match and middleware is skipped as shown in execution_table row 2.

What happens if middleware does not call next()?

The request will hang and the route handler will not execute. In the example, next() is called every time middleware runs (rows 1 and 3), allowing the route handler to run.

Visual Quiz - 3 Questions

Look at the execution table, what is the middlewareExecuted value after the third request?

A false

B true

C N/A

D undefined

💡 Hint

Check the 'middlewareExecuted' row at 'After 3' column in variable_tracker.

At which step does the middleware get skipped because the route does not match?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Route Match' and 'Middleware Action' columns in execution_table.

If we apply middleware to all routes, how would the middlewareExecuted values change?

A All would be false

B All would be true

C Only first and last true

D No change

💡 Hint

Middleware runs on all routes if applied globally, so check variable_tracker for middlewareExecuted.

Concept Snapshot

Define middleware as a class with a use() method.
Apply middleware in a module using consumer.apply().forRoutes(route).
Middleware runs only on matching routes.
Middleware must call next() to continue.
Non-matching routes skip middleware.
Middleware runs before route handler.

Full Transcript

Middleware in NestJS is a function or class that runs before route handlers. You define middleware by creating a class with a use() method that takes request, response, and next. You apply middleware in a module using the configure method and MiddlewareConsumer, specifying which routes it applies to. When a request comes in, NestJS checks if the URL matches the middleware's routes. If yes, the middleware runs, does its work (like logging), then calls next() to pass control to the route handler. If no, middleware is skipped and the route handler runs directly. This process ensures middleware only affects intended routes and runs before the handler.

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