Basics

Boot a Nest application from one module, one controller and one provider, read what Nest prints, and return your first JSON response.

Read route and query parameters, accept a JSON body, await an async service, and override the status code Nest picks for a POST.

Move the logic into an injectable service that keeps state across requests, and answer 404 with NotFoundException when a cat does not exist.

Declare a feature module that owns the cats code, import it from the root module, and export the service so another module shares the same instance.

Validate request bodies with a DTO class and ValidationPipe, strip unknown fields, and let ParseIntPipe turn a route parameter into a number or a 400.

Write a middleware class that records every request to the cats routes and nothing else, and apply it from the module with configure().

Shape every HttpException response with a filter bound to the whole application, and see what the exceptions layer does with an error it does not recognise.

Decide who may call a handler with a guard that reads role metadata off the handler through the Reflector, and answer 401 or 403 depending on why.

Wrap handlers with interceptors: name the handler in a response header on the way in, and map every response body into an envelope on the way out.

Write a parameter decorator that hands handlers the current user or one of its properties, and compose role metadata and a guard into one @Auth() decorator.