What is middleware in NestJS and how does it differ from Express middleware?

Q: What is middleware in NestJS and how does it differ from Express middleware?

@Injectable() export class LoggerMiddleware implements NestMiddleware { constructor(private readonly logService: LogService) {} // DI works here use(req: Request, res: Response, next: NextFunction) { this.logService.log(`${req.method} ${req.url}`); next(); // must call next() or the request will hang } } // Applied in a module @Module({}) export class AppModule implements NestModule { configure(consumer: MiddlewareConsumer) { consumer.apply(LoggerMiddleware).forRoutes('*'); } } Express middleware — plain functions, no DI, registered with app.use(). NestJS class middleware — @Injectable() class, full DI support, applied via configure(). NestJS functional middleware — plain function, no DI, lighter weight for simple cases. Both must call next() to pass control to the next handler or the request will hang. NestJS middleware runs before routing — it has no knowledge of which controller handles the request.

Difficulty: 5/10

NestJS middleware, Express compatibility, request pipeline

NestJS middleware is functionally equivalent to Express middleware — it has access to req, res, and next(). The key difference is that NestJS middleware can be a class implementing NestMiddleware decorated with @Injectable(), giving it full access to the DI container. Functional middleware is still supported for simple stateless cases.

Class-based middleware with DI

NestJS middleware vs Express middleware:

1
Express middleware — plain functions, no DI, registered with app.use().
2
NestJS class middleware — @Injectable() class, full DI support, applied via configure().
3
NestJS functional middleware — plain function, no DI, lighter weight for simple cases.
4
Both must call next() to pass control to the next handler or the request will hang.
5
NestJS middleware runs before routing — it has no knowledge of which controller handles the request.

Scenario Questions

0-2 years experience

1How would you add a logging middleware to a NestJS controller that prints the request method and URL?
2If you place a middleware after the route handler in NestJS, what will happen to the response?
3Can you explain how you would register a middleware that should run for all routes except '/health'?

Follow-up Questions

What are the pros and cons of using NestJS middleware versus plain Express middleware in a large codebase?
How does Nest's dependency injection affect testing of middleware?
Can you describe a scenario where you'd prefer an interceptor over middleware in NestJS?