NestJS: How can I access controller function response using a custom decorator?

This is my decorator

import { createParamDecorator, ExecutionContext } from "@nestjs/common";

export const CacheData = createParamDecorator(
    (data: any, ctx: ExecutionContext) => {
        const request = ctx.switchToHttp().getRequest();
        console.log(request.url, request.method, 'request');
        const response = ctx.switchToHttp().getResponse();
        const reqBody = request.body;
        console.log(reqBody, 'reqBody');
        console.log(response.raw.req.data, 'response');
        console.log(response.raw.req.body, 'response');
        console.log(response.raw.data, 'response');
        console.log(response.cacheData, 'response');
    }
);

in my controller function I'm using the decorator as follows:

getStringsArr(
    @Headers('Authorization') auth: string,
    @Headers('Country') country = 'DK',
    @CacheData() cacheData,
  ): Array<string> {
return ['Hello', 'World', '!'];
}

so how can I access response data in my CacheData decorator?

Your CacheData decorator is a param decorator, which means, as far as I know, that it will be executed only when the method handler is called. You have a few options. Here are two.

Option A - Method decorator

A method decorator would give you access to the returned data from your function but comes with drawbacks: you don't have access to the execution context, unlike the param decorator and injecting customer services is less elegant. I like this option because it is easy to supply parameters to the decorator.

Since your example is around caching, I suspect you'll want to inject your service there, so option B is probably more fitted to your requirements, but here's an implementation with a method decorator :

const MyMethodDecorator = (params) => {
  return (
    target: Record<string, unknown>,
    _propertyKey: string,
    descriptor: PropertyDescriptor,
  ) => {
    const originalMethod = descriptor.value;
    descriptor.value = async function (...args) {
      const data = await originalMethod.apply(this, args);
      // data = ['Hello', 'World', '!'] 
    };

    return descriptor;
  };
};

@MyMethodDecorator({ ttl: 120, cacheKey: 'stringsArr' })
getStringsArr(
  @Headers('Authorization') auth: string,
  @Headers('Country') country = 'DK'
): Array<string> {
  return ['Hello', 'World', '!'];
}

Option B - Route Interceptor

Interceptors make dependency injection easy since it's like any other NestJS service. I recommend reading and understanding the request lifecycle is you choose this option.

One drawback vs a decorator is that supplying parameters is less elegant but is doable using reflection and a method decorator:

import { applyDecorators, SetMetadata, UseInterceptors } from '@nestjs/common';

@Injectable()
export class MyInterceptor implements NestInterceptor {

  constructor(private readonly reflector: Reflector) {}

  intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
    const params = this.reflector.get('MyInterceptorMetadataKey', context.getHandler());
    // params = { ttl: 120, cacheKey: 'stringsArr' }

    return next.handle().pipe(
      tap((response) => {
        // data = ['Hello', 'World', '!'] 
      }),
    );
  }
}

const MyMethodDecorator = (params) => 
    applyDecorators(SetMetadata('MyInterceptorMetadataKey', params));

@UseInterceptors(MyInterceptor)
@MyMethodDecorator({ ttl: 120, cacheKey: 'stringsArr' })
getStringsArr(
  @Headers('Authorization') auth: string,
  @Headers('Country') country = 'DK'
): Array<string> {
  return ['Hello', 'World', '!'];
}