Now without the interface...

structural_typing2.ts

import { expect } from 'chai';
import 'mocha';

class HelloEndpoint {
  constructor(private service: HelloService) {}

  async handle(evt: any): Promise<any> {
    const greeting = await this.service.sayHello(evt.queryStringParameters.name);
    return {
      body: greeting
    };
  }
}

// Concrete implementation
class HelloService {

  async sayHello(name: string): Promise<string> {
    throw Error("Not implemented yet")
  }

}

const helloService: HelloService & { receivedName?: string } = {
  sayHello: async (name: string): Promise<string> => {
    helloService.receivedName = name;
    return `Hello ${name}!`;
  }
}

const helloEndpoint = new HelloEndpoint(helloService);

describe("The Hello Endpoint", () => {
  it("Should greet the person", async () => {
    // Call the endpoint.
    const result = await helloEndpoint.handle({
      queryStringParameters: {
        name: "Arthur Putey"
      }
    });

    // Validate that the right data is passed into the service.
    expect(helloService.receivedName).to.equal("Arthur Putey");

    // Validate that the right response is returned to the client.
    expect(result.body).to.equal("Hello Arthur Putey!");
  });
});