contravariant.ts

type ICallback = (i: number) => string

class Foo {
  cb: ICallback = () => ''
  registerCb(cb: ICallback) {
    this.cb = cb
  }
}

const foo = new Foo()

function correctCb(i: number): string {
  return 'Hello world ' + i ** 2 + ''
}

function wrongCb(): string {
  return ''
}

function wrongCb2(): void {
  return
}

function wrongCb3(s: string): string {
  return '';
}


foo.registerCb(correctCb)
foo.registerCb(wrongCb)
foo.registerCb(wrongCb2)
foo.registerCb(wrongCb3)