interface Person {
  // If age exists, then it must be a number; but it is not required to exist.
  age?: number;
  formatName: () => string;
  name: string;
}

abstract class FooBase {
  age: number;
  name: string;
}

class Foo extends FooBase implements Person {
  constructor(props: string[]) {
    super();
    this.age = 0;
    this.name = "Doe";
  }

  formatName(): string {
    return this.name;
  }
}