Abstract classes are a special type of class that can only be inherited from and not instantiated. They are declared with the abstract keyword, as in the following example:

1. We can define a base Product class as follows:

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

2. If we try to create an instance of this, the compiler will complain, as we would expect:

3. We can create a more specific usable class for food products by extending Product:

class Food extends Product {
  constructor(public bestBefore: Date) {
    super();
  }
}

4. Here, we are adding a bestBefore date in our Food class. We can then create an instance of Food, passing in the bestBefore date:

const bread = new Food(new Date(2019, 6, 1));

Abstract classes can have abstract methods that child classes must implement. Abstract methods are declared with the abstract keyword in front of them, as in the following example:

1. Let's add an abstract method to our base Product class:

abstract class Product {
  name: string;
  unitPrice: number;
  abstract delete(): void;
}

2. After we add the abstract method, the compiler immediately complains about our Food class because it doesn't implement the delete method:

3. So, let's fix this and implement the delete method:

class Food extends Product {
  deleted: boolean;
  
  constructor(public bestBefore: Date) {
    super();
  }
  
  delete() {
    this.deleted = false;
  }
}