第21题：async-promise相关异步知识的考察，必会！

[zxdfe]

• 以下代码输出什么？

  async function async1() {
  console.log('async1 start');
  await async2();
  console.log('async1 end');
  }
  async function async2() {
  console.log('async2');
  }
  console.log('script start');
  setTimeout(function() {
  console.log('setTimeout');
  }, 0)
  async1();
  new Promise(function(resolve) {
  console.log('promise1');
  resolve();
  }).then(function() {
  console.log('promise2');
  });
  console.log('script end');

[Moooodena]

script start async1 start async2 promise1 script end async1 end promise2 setTimeout

[A-Lonely-GEEK]

   'script start'
    async1 start
    async2
    promise1
    script end
    async1 end
    promise2
    setTimeout

[szgyFE]

首先是同步任务依次执行 console.log('script start'); async1()：调用函数执行 console.log('async1 start')和 await async2() awiat会阻塞往下执行，所以执行console.log('async2');继续往下执行同步代码console.log('promise1'); console.log('script end');然后执行await后面的 console.log('async1 end');最后 then 为微任务 先执行console.log('promise2');，setTimeout为宏任务，后执行console.log('setTimeout'); 结果如上