TS4113

This member cannot have an 'override' modifier because it is not declared in the base class 'MyBaseClass'.

Broken Code ❌

class MyBaseClass {
  sayHello(): string {
    return 'Hello!';
  }
}
 
class MyDerivedClass extends MyBaseClass {
  override sayWelcome(): string {
    return 'Welcome!';
  }
}

Fixed Code ✔️

You can only override functions in our derived class when those exist in our base class. We can solve the error by overwriting an existing function:

class MyBaseClass {
  sayHello(): string {
    return 'Hello!';
  }
}
 
class MyDerivedClass extends MyBaseClass {
  override sayHello(): string {
    return 'Welcome!';
  }
}

Depending on our use case, we can also remove the override modifier:

class MyBaseClass {
  sayHello(): string {
    return 'Hello!';
  }
}
 
class MyDerivedClass extends MyBaseClass {
  sayWelcome(): string {
    return 'Welcome!';
  }
}