ForgotPassword.test.ts

describe("ForgotPassword", () => {
  test("it sends email when reset is requested", async () => {
    const users = new UsersInMemory();
    const forgotPassword = new ForgotPassword(users);

    // ...
  });
});

ForgotPassword.ts

export class ForgotPassword {
  constructor(private readonly users: Users) {}

  async requestReset(email: Email) {
    const user = await this.users.getByEmail(email);

    // ...
  }
}

Users.test.ts

function generateTests(instance: () => Users) {
  test("it returns user by email", async () => {
    const users = instance();
    const john = new User(/* ... */);
    await users.persist(john);
    expect(await users.getByEmail(john.getEmail())).toEqual(john);
  });
}

describe("UsersInMemory", () => {
  generateTests(() => new UsersInMemory());
});

describe("UsersMySQL", () => {
  beforeEach(async () => {
    // ...
    await mysql.query("TRUNCATE users");
  });

  generateTests(() => new UsersMySQL(mysql));

  afterEach(() => {
    mysql.close();
  });
});

Users.ts

export interface Users {
  getByEmail(email: Email): Promise<User | undefined>;
  persist(user: User): Promise<void>;
}

export class UsersInMySQL implements Users {
  constructor(private readonly mysql: MySql) {}

  async getByEmail(email: Email) {
    const rows = await this.mysql.select(
      "SELECT id, email, ... FROM users WHERE email = LOWERCASE(:email)",
      { email: email.toString() }
    );

    // ...
  }

  async persist(user: User) {
    await this.mysql.insert("INSERT INTO users (...) VALUES (...) ON DUPLICATE KEY UPDATE ...");
  }

  // ...
}

export class UsersInMemory implements Users {
  private users: User[] = [];

  async getByEmail(email: Email) {
    return this.users.find(/* ... */);
  }

  async persist(user: User) {
    // Simplified
    this.users.push(user);
  }
}