SOLID - Dependency Inversion Principle

dip_ex1.ts

// infra/HttpClient.ts
import axios from "axios";

export default {
  createUser: async (user: User) => {
    return axios.post(/* ... */);
  },
  getUserByEmail: async (email: string) => {
    return axios.get(/* ... */);
  },
};

// domain/SignupService.ts
import HttpClient from "infra/HttpClient"; // ❌ the domain depends on a concretion from the infra

export async function signup(email: string, password: string) {
  const existingUser = await HttpClient.getUserByEmail(email);

  if (existingUser) {
    throw new Error("Email already used");
  }

  return HttpClient.createUser({ email, password });
}

dip_ex2.ts

// domain/ApiClient.ts
export interface ApiClient {
  createUser: (user: User) => Promise<void>;
  getUserByEmail: (email: string) => Promise<User>;
  // ...
}
    
// infra/HttpClient.ts
import axios from "axios";
import ApiClient from "domain/ApiClient";

export function HttpClient(): ApiClient {
  return {
    createUser: async (user: User) => {
      return axios.post(/* ... */);
    },
    getUserByEmail: async (email: string) => {
      return axios.get(/* ... */);
    },
  };
}
    
// domain/SignupService.ts
import ApiClient from "domain/ApiClient"; // ✅ the domain depends on an abstraction of the infra

export function SignupService(client: ApiClient) {
  return async (email: string, password: string) => {
    const existingUser = await client.getUserByEmail(email);

    if (existingUser) {
      throw new Error("Email already used");
    }

    return client.createUser({ email, password });
  };
}

// index.ts -> With the power of dependency inversion, our SignupService can now use any ApiClient. Let's inject our HttpClient for now:
import SignupService from "domain/signup";
import HttpClient from "infra/HttpClient";

const signup = SignupService(HttpClient());

signup("bob@bob.com", "pwd123");