To keep it brief:

I have a service - let's call it CatsService...

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

@Injectable()
export class CatsService {
    GetCat(id: number){
        return id + 12345;
    }
}

Later on, a module wishes to use this CatsService in order to inject it into a constructor. It also dynamically resolves this CatsService

import { Module } from '@nestjs/common';
import { CatsController } from './cats.controller';
import { CatsService } from './cats.service';
import * as ProductionCatsService from "./production"
import * as DevelopmentCatsService from "./development"

const catsServiceProvider = {
  provide: CatsService,
  useValue: process.env.NODE_ENV === "development" ? 
    DevelopmentCatsService  :
    ProductionCatsService
}

@Module({
  controllers: [CatsController],
  providers: [catsServiceProvider],
  exports:[CatsService]
})
export class CatsModule {}

Both of these service implementations are strictly empty. This means that the /cats/:id route returns:

{"statusCode":500,"message":"Internal server error"}

I have had the idea since beginning writing this to make everything implement a ICatsService interface.

After a lot of fiddling about, I have build a sort-of solution:

1. I have a "service_interface" file, which has an abstract class called ICatsInterface with one abstract method (GetCat)

2. I have made every one of these implementations implement this service_interface.

3. I have made the controller take ICatsService instead of CatsService

My problem is that now, the thing compiles, but I get the below error:

TypeError: this.catsService.GetCat is not a function
    at CatsController.get (/home/a/learning-nest/src/cats/cats.controller.ts:12:33)
    at /home/a/learning-nest/node_modules/@nestjs/core/router/router-execution-context.js:38:29
    at processTicksAndRejections (node:internal/process/task_queues:96:5)
    at /home/a/learning-nest/node_modules/@nestjs/core/router/router-execution-context.js:46:28
    at /home/a/learning-nest/node_modules/@nestjs/core/router/router-proxy.js:9:17

I simply don't know what to do anymore. Even though I have made it compile, I cannot make it actually be forced to implement the interface/abstract class that I want. I have made sure that both DevelopmentCatsService and ProductionCatsService have the implementation of GetCat. They are both marked with @Injectable. I don't truthfully know where to go from here.

Should I abandon my dream of having an compile-time error message for when someone does not implement the methods that I want implemented in my service?