nullCheck.ts

// Adapted from Code Smell: Null Check, Joe Eames https://medium.com/thinkster-io/code-smell-null-check-a0c4851fafbf

const DEFAULT_DISCOUNT: number = 1.0;

class DiscountCalculator {
  private customerRepository: CustomerRepository;

  constructor(customerRepository: CustomerRepository) {
    this.customerRepository = customerRepository;
  }

  // ...
  getDiscount(customerId: string): number {
    const customer = this.customerRepository.findBy(customerId);
    if (customer == null) {
      return DEFAULT_DISCOUNT;
    }
    return customer.getEarnedDiscount();
  }
}

class Orders {
  private customerRepository: CustomerRepository;

  constructor(customerRepository: CustomerRepository) {
    this.customerRepository = customerRepository;
  }

  // ...
  processOrder(customerId: string, order: Order): void {
    const customer = this.customerRepository.findBy(customerId);
    if (customer != null) {
      customer.addToOrdersHistory(order);
    }
    // La otra versión
    // customer?.addToOrdersHistory(order);
    // more logic ...
  }
}


//--------------------------------------------------------------------
//--------------------------------------------------------------------

interface Customer {
  getEarnedDiscount(): number;

  addToOrdersHistory(order: Order): void;
}

// some Customer's implementations
// ...

// Null Customer's implementation
class NotFoundCustomer implements Customer {
  private DEFAULT_DISCOUNT: number = 1.0;

  //...
  public getEarnedDiscount(): number {
    return this.DEFAULT_DISCOUNT;
  }

  public addToOrdersHistory(order: Order): void {

  }
}

class DiscountCalculator {
  private customerRepository: CustomerRepository;

  constructor(customerRepository: CustomerRepository) {
    this.customerRepository = customerRepository;
  }

  // ...
  getDiscount(customerId: string): number {
    const customer = this.customerRepository.findBy(customerId);
    return customer.getEarnedDiscount();
  }
}

class Orders {
  private customerRepository: CustomerRepository;

  constructor(customerRepository: CustomerRepository) {
    this.customerRepository = customerRepository;
  }

  // ...
  processOrder(customerId: string, order: Order): void {
    const customer = this.customerRepository.findBy(customerId);
    customer.addToOrdersHistory(order);

    // more logic ...
  }
}