CODESAMPLE

Null Object - TypeScript

Null Object TypeScript

The Null Object pattern provides a substitute object for null references, allowing default or no-op behavior instead of throwing exceptions when attempting to operate on a null value. This improves code readability and reduces conditional checks for null.

The TypeScript example implements a Vehicle interface with methods like getMileage(). A NullVehicle class implements the same interface, providing default values (0 for mileage) and no-op behavior for other methods. Instead of returning null when a vehicle is unavailable, the code returns an instance of NullVehicle. This allows calling vehicle.getMileage() without needing a null check, simplifying the client code. Using interfaces and classes is a standard TypeScript practice for defining types and structures.

// Vehicle Interface
interface Vehicle {
  getMileage(): number;
  getType(): string;
}

// Concrete Vehicle Class
class Car implements Vehicle {
  constructor(private mileage: number) {}

  getMileage(): number {
    return this.mileage;
  }

  getType(): string {
    return "Car";
  }
}

// Null Vehicle Class - the Null Object
class NullVehicle implements Vehicle {
  getMileage(): number {
    return 0;
  }

  getType(): string {
    return "Unknown";
  }
}

// Function that might return a vehicle or a null object
function getVehicle(hasVehicle: boolean): Vehicle {
  if (hasVehicle) {
    return new Car(10000);
  } else {
    return new NullVehicle();
  }
}

// Client Code - no null checks needed!
const vehicle1 = getVehicle(true);
console.log(`Vehicle 1 Mileage: ${vehicle1.getMileage()}, Type: ${vehicle1.getType()}`);

const vehicle2 = getVehicle(false);
console.log(`Vehicle 2 Mileage: ${vehicle2.getMileage()}, Type: ${vehicle2.getType()}`);