function* 这种声明方式(function关键字后跟一个星号）会定义一个生成器函数 (generator function)，它返回一个  Generator  对象。

function* generator(i) {
  yield i;
  yield i + 10;
}

const gen = generator(10);

console.log(gen.next().value);
// expected output: 10

console.log(gen.next().value);
// expected output: 20
console.log(gen.next().value);
// expected output: undefined
console.log(gen.next().value);
// expected output: undefined