Alternative using Parameterize Constructor

IntroduceInstanceDelegator2.ts

// Alternative using Parameterize Constructor

export class User {
    private readonly id: number;
    private readonly bankingServices: BankingServices;

    //!! We used Parameterize Constructor to inject a BankingServices instance to User
    constructor(id: number, bankingServices: BankingServices) {
        this.id = id;
        this.bankingServices = bankingServices;
    }

    // more code...

    public updateBalance(amount: Money): void {
        this.bankingServices.updateBalance(this.id, amount);
    }

    // more code...
}

//!! In some test now we can use a test double of BankingServices to sense or separate
describe("User", () => {
    it('updates balance', () => {
        const bankingServices: jest.Mocked<BankingServices> = { updateBalance: jest.fn() };
        const userId = 1;
        const user = new User(userId, bankingServices);
        const amount = new Money(200);

        user.updateBalance(amount);

        expect(bankingServices.updateBalance).toHaveBeenCalledWith(userId, amount); // in this case sensing
    });

    // more tests...

});