function *generatorify(dog) {
  while (dog.barkCount < 3) {
    yield dog.bark()
  }
}

class chihuahua {
  constructor () {
  this.barkCount = 0
  }
  bark () {
    this.barkCount++
    return 'yap'
  }
}

const caesar = new chihuahua()
const barkALot = generatorify(caesar)

console.log(barkALot.next().value, caesar.barkCount) // 'yap', 1
console.log(barkALot.next().value, caesar.barkCount) // 'yap', 2
console.log(barkALot.next().value, caesar.barkCount) // 'yap', 3
console.log(barkALot.next().done, caesar.barkCount)  // true, 3