关于事件循环机制，一个例子

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

async function async2 () {
  console.log('async2')
  setTimeout(function () {
    console.log('async2 setTimeout')
  }, 0)
}

async function async3 () {
  console.log('async3')
}

console.log('script start')

setTimeout(async function () {
  console.log('setTimeout1 start')
  await async3()
  console.log('setTimeout1 end')
}, 0)

async1().then(function () {
  console.log('async1 after')
}).then(function () {
  console.log('async1 after2')
})

new Promise(function (resolve) {
  console.log('promise1')
  resolve()
}).then(function () {
  console.log('promise2')
}).then(function () {
  console.log('promise3')
})

console.log('script end')

output

script start
async1 start
async2
promise1
script end
async1 end
promise2
async1 after
promise3
async1 after2
setTimeout1 start
async3
setTimeout1 end
async2 setTimeout