In nest.js, is it possible to get service instance inside a param decorator?

for NestJS v7

Create custom pipe

// parse-token.pipe.ts
import { ArgumentMetadata, Injectable, PipeTransform } from '@nestjs/common';
import { AuthService } from './auth.service';

@Injectable()
export class ParseTokenPipe implements PipeTransform {
    // inject any dependency
    constructor(private authService: AuthService) {}
    
    async transform(value: any, metadata: ArgumentMetadata) {
        console.log('additional options', metadata.data);
        return this.authService.parse(value);
    }
}

Use this pipe with property decorator

// decorators.ts
import { createParamDecorator, ExecutionContext } from '@nestjs/common';
import { ParseTokenPipe} from './parse-token.pipe';

export const GetToken = createParamDecorator((data: unknown, ctx: ExecutionContext) => {
  return ctx.switchToHttp().getRequest().header.Authorization;
});

export const Principal = (additionalOptions?: any) => GetToken(additionalOptions, ParseTokenPipe);

Use this decorator with or without additional options

@Controller('/test')
class TestController {
  @Get()
  get(@Principal({hello: "world"}) principal) {}
}

It is not possible to inject a service into your custom decorator.

Instead, you can create an AuthGuard that has access to your service. The guard can then add a property to the request object, which you can then access with your custom decorator:

@Injectable()
export class AuthGuard implements CanActivate {
  constructor(private authService: AuthService) {}

  async canActivate(context: ExecutionContext): Promise<boolean> {
    const request = context.switchToHttp().getRequest();
    const bearerToken = request.header.Authorization;
    const user = await this.authService.authenticate(bearerToken);
    request.principal = user;
    // If you want to allow the request even if auth fails, always return true
    return !!user;
  }
}

import { createParamDecorator } from '@nestjs/common';

export const Principal = createParamDecorator((data: string, req) => {
  return req.principal;
});

and then in your controller:

@Get()
@UseGuards(AuthGuard)
get(@Principal() principal: Principal) {
  // ...
}

---

Note that nest offers some standard modules for authentication, see the docs.