首先看一段代码：

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

打印顺序是什么？ 正确答案是 script start, script end, promise1, promise2, setTimeout