Choose a Superclass or Interface for your Subclass

I currently have the following setup:

class Dog extends Animal{}

class Animal{}

This is a simple inheritance structure. Now, I have another class that contains all types of animals.

class Farm{
     animals: Animal[];
}

At a certain point in my application, I need to loop through the array of animals. When I come across a dog, I want to assign it to a variable:

_.forEach(farm.animals, (animal:Animal)=>{
    if(animal instanceof Dog){
        this.dog = animal;
    }
}

However, I am encountering two errors. Firstly, the 'instanceof' operator is not functioning properly and showing an error stating that the right-hand side of the argument is null.

The second error states that type 'Animal' is not assignable to type 'Dog'. The same issue occurs when I declare 'Animal' as an interface.

Answer №1

It seems like the instanceof error is just a small mistake, as actually using instanceof is correct in this case.

The reason for not being able to assign this.dog = animal; is because you need to perform a type cast (strangely, I couldn't replicate this error in the TypeScript playground):

this.dog = <Dog>animal;

Alternatively, you can use:

this.dog = animal as Dog;

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