Extending classes

Classes can extend other classes. This is the same concept as interfaces extending other interfaces, which we covered in the Extending interfaces section. This is a way for class properties and methods to be shared with child classes.

As with interfaces, we use the extends keyword followed by the class we are extending. Let's look at an example:

1. Let's create a ProductWithDiscountCodes from our Product class:

class Product {
  name: string;
  unitPrice: number;
}

interface DiscountCode {
  code: string;
  percentage: number;
}

class ProductWithDiscountCodes extends Product {
  discountCodes: DiscountCode[];
}

1. We can then consume the ProductWithDiscountCodes class as follows, leveraging properties from the base class as well as the child class:

const table = new ProductWithDiscountCodes();
table.name = "Table";
table.unitPrice = 500;
table.discountCodes = [
  { code: "SUMMER10", percentage: 0.1 },
  { code: "BFRI", percentage: 0.2 }
];

1. If the parent class has a constructor, then the child class will need to pass the constructor parameters using a function called super:

class Product {
  constructor(public name: string, public unitPrice: number) {
  }
}

interface DiscountCode {
  code: string;
  percentage: number;
}

class ProductWithDiscountCodes extends Product {
  constructor(public name: string, public unitPrice: number) {
    super(name, unitPrice);
  }
  discountCodes: DiscountCode[];
}