javascript - 将 Promise 重构为 Async/await。为什么我的未定义？

Question

我有 Promise 上的代码:

function one() {
  return new Promise((rs,er)=>{
  console.log('start');
  new Promise((rr,err)=>{setTimeout(()=>{console.log('after 3 sec');rr()},3000)})
    .then( sd => {
      console.log('endLong');
      rs();
      })
    });
}

one().then(_ => one()).then(_ => one());

结果我得到了同步代码。

但是当我将其重构为 async/wait

async function one() {
  console.log('start');
  await longtime();
  console.log('endLong');
}

async function longtime() {setTimeout(()=>{console.log('after 3 sec');},3000);}

one().then(_ => one()).then(_ => one())

然后我得到:

start
endLong
start
endLong
start
endLong
Promise {<resolved>: undefined}
after 3 sec
after 3 sec
after 3 sec

出了什么问题？

Answer 1

您需要长时间才能返回 promise 。单独的 setTimeout 对于基于 Promise 的 async/await 来说没有任何意义:

async function one() {
  console.log('start');
  await longtime();
  console.log('endLong');
}

const longtime = () => new Promise((resolve, reject) => {
  setTimeout(() => {
    console.log('after 0.5 sec');
    resolve();
  }, 500);
});

one()
  .then(_ => one())
  .then(_ => one())