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1What can middleware NOT do that guards or interceptors can in NestJS?
2What is middleware in NestJS and how does it differ from Express middleware?
3How do you apply middleware to specific routes or exclude certain routes in NestJS?
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What is middleware in NestJS and how does it differ from Express middleware?

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. 1

    Express middleware — plain functions, no DI, registered with app.use().

  2. 2

    NestJS class middleware — @Injectable() class, full DI support, applied via configure().

  3. 3

    NestJS functional middleware — plain function, no DI, lighter weight for simple cases.

  4. 4

    Both must call next() to pass control to the next handler or the request will hang.

  5. 5

    NestJS middleware runs before routing — it has no knowledge of which controller handles the request.