CODEWITHSUNDEEP
typescriptinheritanceparent-child
Ilja
Ilja

Reputation: 46509

Possibility to override (change args) of a super class method

I came across following issue, I have 2 classes i.e.

class Animal {
  public talk() {
    console.log('...');
  }
}

and

class Dog extends Animal {
  public talk(noise: string) {
    console.log(noise);
    super.talk()
  }
}

As you can see original Animal.talk method has no arguments and due to constraints of my application it can't have any, however some classes that extend it can take in additional noise argument. This throws a type error since this argument is not initially on Animal. Hence I wanted to ask how to handle this correctly.

This is a very simplified example, real use case has various types of noise and I'd like to avoid defining all of these as possible values of Animal.talk and ideally keep it without arguments;

Upvotes: 1

Views: 61

Answers (1)

Rob Napier
Rob Napier

Reputation: 299455

It is required that Dog accept both talk("string") and talk() in this case. If you pass a Dog to a function that takes an Animal, that function must be able to call animal.talk() (or else you've violated substitutability). That means that noise has to be optional:

class Dog extends Animal {
  public talk(noise?: string) {
    console.log(noise);
    super.talk()
  }
}

And then you just need to then deal with cases where noise is not passed. (But that's not avoidable. You will always have to deal with cases where noise is not passed.)

Upvotes: 1

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