事件循环[话题]

1. 什么是事件循环？
2. 事件循环和宿主环境有关系吗？浏览器和node环境区别？
3. 宏任务与微任务？
4. 分析下面代码的执行顺序，能否解释为什么？

   new Promise((resolve, reject) => {
   console.log(1);
   setTimeout(() => {
    console.log(2)
    resolve(2);
   }, 0);
   console.log(3)
   })

   ```javascript 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(‘setTimeout0’) },0)
   setTimeout(function(){ console.log(‘setTimeout3’) },3)
   setImmediate(() => console.log(‘setImmediate’)); process.nextTick(() => console.log(‘nextTick’)); async1(); new Promise(function(resolve){ console.log(‘promise1’) resolve(); console.log(‘promise2’) }).then(function(){ console.log(‘promise3’) }) console.log(‘script end’)

```