nullCheck.js

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

const DEFAULT_DISCOUNT = 1.0;

class DiscountCalculator {
  constructor(customerRepository) {
    this.customerRepository = customerRepository;
  }

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

class Orders {
  constructor(customerRepository) {
    this.customerRepository = customerRepository;
  }

  // ...
  processOrder(customerId, order) {
    const customer = this.customerRepository.findBy(customerId);
    if (customer != null) {
      customer.addToOrdersHistory(order);
    }
    // more logic ...
  }
}


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

// The interface of Customer objects has two methods in it (shown in pseudocode):
// * getEarnedDiscount(): number;
// * addToOrdersHistory(order: Order): void;

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

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

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

  public addToOrdersHistory(order: Order): void {

  }
}

class DiscountCalculator {
  constructor(customerRepository) {
    this.customerRepository = customerRepository;
  }

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

class Orders {
  constructor(customerRepository) {
    this.customerRepository = customerRepository;
  }

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

    // more logic ...
  }
}