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.