async function async1() {
   console.log('async1 start')
   await async2()
   console.log('async1 end')
  }
  async function async2() {
   console.log('async2')
  }
  async1()
  console.log('script start')

   // js是单线程的 是非阻塞的 我们去掉await 再看结果

 function async1() {
   console.log('async1 start')
    async2()
   console.log('async1 end')
  }
   function async2() {
   console.log('async2')
  }
  async1()
  console.log('script start')

 结果符合我们的预期。