实现Promise的串行 ​

async function serial(...args) {
  return args.reduce((task, now) => task.then(res => now), Promise.resolve())
}

async function serial(...args) {
  return args.reduce((task, now) => task.then(res => now), Promise.resolve())
}

serial(promise1, promise2, ...)

serial(promise1, promise2, ...)