Javascript class interface implements

interface.jsx

class Foo {
  on(foo: string) {
  }
}

interface IBar {
  on(foo: 'bar'): void;
}

class Bar extends Foo implements IBar {
  on(foo: string) {
  }
}

const bar: IBar = new Bar();
bar.on('bar'); //Flow OK
bar.on('foo'); // Flow Error