قراءة سياق الطلب باستخدام Param Decorators

In NestJS controllers, you often need to pull the same piece of data out of the request over and over: the authenticated user, the client IP, a tenant id from a header. Doing this with @Req() and digging into request.user in every handler is repetitive and leaks framework details into your business logic.

Custom param decorators let you encapsulate that extraction once and reuse it everywhere:

• @CurrentUser() instead of req.user
• @ClientIp() instead of parsing x-forwarded-for

The result is cleaner, more testable, and more declarative controller code.

Here is what you usually start with: grabbing the whole request object and reaching into it manually. It works, but every handler repeats the same lines, and the controller now knows about request.user, which is an implementation detail of your auth guard.

Notice how the actual route logic is buried under plumbing. This is exactly what a param decorator removes.

NestJS exposes the factory createParamDecorator from @nestjs/common. You give it a function that receives two arguments and returns whatever value you want injected into the handler parameter.

• data — the optional argument passed when the decorator is used, e.g. @CurrentUser('email').
• ctx: ExecutionContext — a wrapper around the current request context, independent of the transport (HTTP, RPC, WebSocket).

The returned value becomes the parameter's value at call time.

For HTTP apps you convert the generic ExecutionContext into an HTTP-specific argument host and read the request from it:

• ctx.switchToHttp() returns an HttpArgumentsHost.
• .getRequest() gives you the underlying request (Express or Fastify).

Using switchToHttp() keeps the decorator explicit about which transport it targets. The same context could also be switched to RPC or WS for other protocols.

Assume an auth guard (e.g. a JWT strategy) has already attached the authenticated user to request.user. The @CurrentUser() decorator simply returns it.

This is the canonical pattern in enterprise NestJS apps: the guard does authentication, and the decorator gives ergonomic access to the result without exposing the request object.

The first parameter, data, is what the caller passes inside the decorator's parentheses. You can use it to return a single property instead of the whole object:

• @CurrentUser() returns the full user.
• @CurrentUser('email') returns just the email.

Type the data argument as keyof AuthUser so the caller gets autocomplete and compile-time safety on the property name.

Now the controller reads beautifully. The guard guarantees the user exists; the decorator injects exactly what each handler needs. No @Req(), no manual property digging.

This separation is what makes the handler easy to unit-test: you just call the method with a plain user object.

Behind a load balancer or reverse proxy, the real client IP is not request.ip but the first entry of the x-forwarded-for header. A @ClientIp() decorator centralizes this logic so every handler reads the correct address.

Important: only trust x-forwarded-for when you actually run behind a trusted proxy, and enable Express's trust proxy setting. Otherwise clients can spoof the header.

The valuable part of a param decorator is its pure extraction logic. You can lift that into a plain function, unit-test it with a fake request, and call it from the decorator. Here is the IP-parsing logic as a standalone, runnable program.

This demonstrates the rule for choosing the first forwarded address and the fallback behavior — no NestJS or server required to verify it.

Param decorators compose freely. A single handler can mix built-in decorators (@Body, @Param) with your custom ones. NestJS resolves each parameter independently by its decorator metadata.

Here an audit endpoint records who did what and from where, reading the user and IP declaratively.

A custom param decorator returns a raw value, so you can still attach pipes to it just like built-in decorators. Pass a pipe as an extra argument when using the decorator.

• @CurrentUser('id', ParseUUIDPipe) validates that the extracted id is a UUID.
• Pipes run after the decorator factory returns its value.

This lets you keep extraction and validation cleanly separated while still benefiting from NestJS's pipe pipeline.

Test your understanding of how a custom param decorator reads the request.

You learned to read request context declaratively with custom param decorators:

• createParamDecorator((data, ctx) => ...) builds a reusable decorator; the returned value is injected into the handler parameter.
• ctx.switchToHttp().getRequest() retrieves the HTTP request in a transport-explicit way.
• @CurrentUser() wraps request.user (populated by your auth guard) and can return a single property via the data argument typed as keyof AuthUser.
• @ClientIp() centralizes x-forwarded-for parsing, with a fallback to request.ip — trust the header only behind a real proxy.
• Keep extraction logic pure so it is easy to unit-test, and remember you can still chain pipes like ParseUUIDPipe onto your custom decorators.