npm install @nestjsplus/dyn-schematics -g

nest g -c @nestjsplus/dyn-schematics dynpkg <NAME>

? Generate a testing client? Yes

|   .npmignore
|   .prettierrc
|   nest-cli.json
|   package.json
|   README.md
|   tsconfig.build.json
|   tsconfig.json
|   tslint.json
|
\---src
    |   constants.ts
    |   index.ts
    |   mailchimp-habr.module.ts
    |   mailchimp-habr.providers.ts
    |   mailchimp-habr.service.ts
    |   main.ts
    |
    +---interfaces
    |       index.ts
    |       mailchimp-habr-module-async-options.interface.ts
    |       mailchimp-habr-options-factory.interface.ts
    |       mailchimp-habr-options.interface.ts
    |
    \---mailchimp-habr-client
            mailchimp-habr-client.controller.ts
            mailchimp-habr-client.module.ts

npm i --save @mailchimp/mailchimp_transactional

//mailchimp-habr-options.interface.ts
export type MailchimpHabrOptions = string;

//mailchimp-habr-options-factory.interface.ts
import { MailchimpHabrOptions } from './mailchimp-habr-options.interface';
export interface MailchimpHabrOptionsFactory {
createMailchimpHabrOptions(): | Promise<MailchimpHabrOptions> | MailchimpHabrOptions;
}

//mailchimp-habr-module-async-options.interface.ts
import { ModuleMetadata, Type } from '@nestjs/common/interfaces';
import { MailchimpHabrOptionsFactory } from './mailchimp-habr-options-factory.interface';
import { MailchimpHabrOptions } from './mailchimp-habr-options.interface';
export interface MailchimpHabrAsyncOptions
extends Pick<ModuleMetadata, 'imports'> {
inject?: any[];
useExisting?: Type<MailchimpHabrOptionsFactory>;
useClass?: Type<MailchimpHabrOptionsFactory>;
useFactory?: (...args: any[]) => Promise<MailchimpHabrOptions> | MailchimpHabrOptions;
}

// constants.ts
export const MAILCHIMP_HABR_OPTIONS = Symbol('MAILCHIMP_HABR_OPTIONS');
export const MAILCHIMP_HABR_TOKEN = Symbol('MAILCHIMP_HABR_TOKEN');

//mailchimp-habr.service.ts
import { Injectable, Inject } from '@nestjs/common';
import { MAILCHIMP_HABR_OPTIONS} from './constants';
import { MailchimpHabrOptions } from './interfaces';
import * as Mailchimp from '@mailchimp/mailchimp_transactional/src';
interface IMailchimpHabrService {
getInstance(): Mailchimp;
}
@Injectable()
export class MailchimpHabrService implements IMailchimpHabrService {
private serviceInstance: Mailchimp;
constructor(
   @Inject(MAILCHIMP_HABR_OPTIONS) private mailchimpHabrOptions: MailchimpHabrOptions,
) {}
async getInstance(): Mailchimp {
   return this.serviceInstance ? this.serviceInstance : Mailchimp(this.mailchimpHabrOptions);
}
}

//mailchimp-habr.providers.ts
import { MAILCHIMP_HABR_OPTIONS } from './constants';
import { MailchimpHabrOptions } from './interfaces';
export function createMailchimpHabrProviders(options: MailchimpHabrOptions) {
return [
   {
     provide: MAILCHIMP_HABR_OPTIONS,
     useValue: options,
   },
];
}

export class MailchimpHabrModule {
  public static register(options: MailchimpHabrOptions): DynamicModule {
   return {
     module: MailchimpHabrModule,
     providers: createMailchimpHabrProviders(options),
   };
  }
}

// mailchimp-habr.providers.ts
export const MailchimpProvider: Provider = {
provide: MAILCHIMP_HABR_TOKEN,
useFactory: async (mailchimpService) => mailchimpService.getInstance(),
inject: [MailchimpHabrService],
};

@Global()
@Module({
providers: [MailchimpProvider, MailchimpHabrService],
exports: [MailchimpProvider, MailchimpHabrService],
})
export class MailchimpHabrModule {
  /* */
}

//mailchimp-habr.decorator.ts
import { Inject } from '@nestjs/common';
import { MAILCHIMP_HABR_TOKEN } from './constants';
export const InjectMailchimp = () => Inject(MAILCHIMP_HABR_TOKEN);

import { Module } from '@nestjs/common';
import { MailchimpHabrClientController } from './mailchimp-habr-client.controller';
import { MailchimpHabrModule } from '../mailchimp-habr.module';
@Module({
controllers: [MailchimpHabrClientController],
imports: [MailchimpHabrModule.forRoot('YOUR_API_KEY')],
})
export class MailchimpHabrClientModule {}

import { Controller, Get } from '@nestjs/common';
import { InjectMailchimp } from '../mailchimp-habr.decorator';
@Controller()
export class MailchimpHabrClientController {
constructor(@InjectMailchimp() private readonly mailchimpHabrService) {}
@Get()
index() {
   return this.mailchimpHabrService.users.ping();
}
}

npm run start:dev

import { Module } from '@nestjs/common';
import { MailchimpHabrClientController } from './mailchimp-habr-client.controller';
import { MailchimpHabrModule } from '../mailchimp-habr.module';
@Module({
controllers: [MailchimpHabrClientController],
imports: [MailchimpHabrModule.forRootAsync({ useFactory: () => 'API_KEY' })],
})
export class MailchimpHabrClientModule {}

import { Module } from '@nestjs/common';
import { ConfigModule, ConfigService } from '@nestjs/config';
import { MailchimpHabrModule } from '../mailchimp-habr.module';
import { MailchimpHabrClientController } from './mailchimp-habr-client.controller';
@Module({
controllers: [MailchimpHabrClientController],
imports: [
   ConfigModule.forRoot(),
   MailchimpHabrModule.forRootAsync({
   imports: [ConfigModule],
   useFactory: async (config: ConfigService) => {
     return config.get('API_KEY');
   },
   inject: [ConfigService],
})],
})
export class MailchimpHabrClientModule {}

export class MailchimpHabrModule {
  public static forRootAsync(options: MailchimpHabrAsyncOptions,): DynamicModule {
   return {
     module: MailchimpHabrModule,
     imports: options.imports ?? [], // Добавим imports
     providers: [
       ...this.createProviders(options),
     ],
   };
  }
}

import * as Mailchimp from '@mailchimp/mailchimp_transactional/src';
import { Test } from '@nestjs/testing';
import { MAILCHIMP_HABR_TOKEN, MailchimpHabrModule } from './';
describe('Mailchimp forRoot', () => {
let mailchimpService: Mailchimp;
beforeEach(async () => {
   const moduleRef = await Test.createTestingModule({
     imports: [MailchimpHabrModule.forRoot('MAILCHIPM_API_KEY')],
   }).compile();
   mailchimpService = moduleRef.get<Mailchimp>(MAILCHIMP_HABR_TOKEN);
});
describe('mailchimpHabrService', () => {
   it('method ping exists', async () => {
     expect(mailchimpService.users.ping).toBeDefined();
   });
   it('method is a function', async () => {
     expect(mailchimpService.users.ping).toBeInstanceOf(Function);
   });
   it('users.ping() should return "PONG"', async () => {
     const result = 'PONG';
     jest
       .spyOn(mailchimpService.users, 'ping')
       .mockImplementation(() => result);
     expect(await mailchimpService.users.ping()).toBe(result);
   });
});
});