Creating middleware in NestJS - Quick Revision & Summary

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 objects or end the request-response cycle.

beginner

How do you create a middleware class in NestJS?

Create a class that implements the <code>NestMiddleware</code> interface and define a <code>use(req, res, next)</code> method.

intermediate

How do you apply middleware to routes in NestJS?

Use the configure() method in a module that implements NestModule and call consumer.apply(MiddlewareClass).forRoutes('route').

beginner

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

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

intermediate

Can NestJS middleware be asynchronous? How?

Yes, middleware can be asynchronous by making the use() method async and using await inside it before calling next().

Which interface must a NestJS middleware class implement?

A MiddlewareHandler

B MiddlewareInterface

C NestMiddleware

D RouteMiddleware

NestJS middleware classes must implement the NestMiddleware interface to define the use() method.

Where do you apply middleware in a NestJS application?

A Inside a service class

B Directly inside a controller method

C In the main.ts bootstrap file only

D In the configure() method of a module implementing NestModule

Middleware is applied in the configure() method of a module that implements NestModule using the consumer object.

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

A The request will hang and not reach the route handler

B The request will skip the middleware

C The server will crash

D The middleware will run twice

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

Which of these is NOT a valid way to apply middleware in NestJS?

A Adding middleware directly in the controller decorator

B consumer.apply(MyMiddleware).exclude('dogs').forRoutes('cats')

C consumer.apply(MyMiddleware).forRoutes('cats')

D Applying middleware globally in main.ts

Middleware cannot be added directly in controller decorators; it must be applied via the module's configure() or globally.

Can middleware modify the response object before sending it?

A No, middleware can only read the request

B Yes, middleware can modify req and res objects

C No, only interceptors can modify responses

D Yes, but only if it returns a value

Middleware can modify both request and response objects before passing control along.

Explain how to create and apply middleware in a NestJS module.

Think about the class, interface, and where you connect middleware to routes.

Describe the role of the next() function in NestJS middleware and what happens if it is not called.

Consider how middleware controls the flow of a request.