Typescript not recognizing properties on the parent class?

Question

I have a basic inheritence like so:

export abstract class Animal
{
     constructor() {

     }

     makeSound() {
         alert(this.sound);
     }
}

export class Dog extends Animal
{
     public sound: string = "Woof!";

     constructor() {
         super();
     }
}

const dog = new Dog;
dog.makeSound(); // Woof!

I'm having an issue trying to get the above snippet working. Property prop does not exist on type Animal. However, it does exist on Dog, and since it is public, Animal should be able to reach prop via this.prop.

I tried the following:

• Same as above - It doesn't compile. (Property prop does not exist on type Animal)
• If I add public prop: any; to Animal, then prop becomes undefined and not Hello world.
• If I add public abstract prop: any; to Animal, same thing happens.

The whole point of inheritence by using abstract classes is to share this scope.

How can I do it properly with Typescript?

Answer 1

I agree with JB Nizet. But you also have the ability to define sound as an abstract property. That way your base class knows about sound, and all extending classes have to implement the abstract property sound:

export abstract class Animal
{
    abstract sound: string; // Only change here, now your code works
    constructor() {

    }

    makeSound() {
        alert(this.sound);
    }
}

export class Dog extends Animal {
    public sound: string = "Woof!";

    constructor() {
        super();
    }
}

const dog = new Dog;
dog.makeSound(); // Woof!

The ability to define abstract properties came with TS 2.0: https://github.com/Microsoft/TypeScript/issues/4669

Update

The problem OP experienced with this approach, see comments, was that the abstract property was used in the constructor of the abstract base class. This will not work since the property is assigned the initial value from the deriving class first after the object is instantiated.

There is a github issue for this problem, where the solution is to simply error on access to abstract properties within the constructor of the abstract class.