Extend NestJS decorator

Question

I stumbled across this question but I don't think I want to use an alias

I want to extend the express anyFilesInterceptor so I can work with a custom file object. I am not sure how to extend a decorator in NestJS.

So as a work around I tried decorator composition from another question. However, I am getting an error just trying to create a very basic (example in documentation) decorator

import { applyDecorators, createParamDecorator, ExecutionContext } from "@nestjs/common";
import { AnyFilesInterceptor } from "@nestjs/platform-express";

export function Test() {
  return applyDecorators(
    AnyFilesInterceptor,
    TestDecorator
  )
}

export const TestDecorator = createParamDecorator(
  (data: string, ctx: ExecutionContext) => {
    const request = ctx.switchToHttp().getRequest();
    const user = request.user;

    return data ? user?.[data] : user;
  },
);

Now I can see from other discussions and the function naming that AnyFilesInterceptor is a mixin that returns a class while TestDecorator created by createParamDecorator likely only works on parameters.

Does NestJS have a way to create a class decorator? Or to extend existing decorators?

Answer 1

actually the AnyFilesInterceptor is a function itself that produces an interceptor (which is any class that implements NestInterceptor).
You can see it by the usage: while 'other' interceptors may be used by simply giving the class to the UseInterceptor() decorator, this interceptor needs invocation (without new keyword).
Example:

@UseInterceptor(RegularInterceptor)
//or
@UseInterceptor(new RegularInterceptor())

// AnyFilesInterceptor is a function returning a class
@UseInterceptor(AnyFilesInterceptor())
//or
@UseInterceptor(new (AnyFilesInterceptor())({/* some multer options here */))

so basically if you want to extend the AnyFilesInterceptor you simply need to define a class inteceptor of your own:

export class MyAllFilesInterceptor extends AnyFilesInterceptor() {
  // YOU MUST OVERRIDE THE `intercept` METHOD!
  // also, give options to the `AnyFilesInterceptor` method if you wish
}