js 事件循环（Event Loop）机制

先放个测试题，压压惊

console.log('start');

const interval = setInterval(()=>{
  console.log('setInterval');
},0);

setTimeout(()=>{
  console.log('setTimeout 1');
  Promise.resolve()
    .then(()=>{
      console.log('promise1');  
    })
    .then(()=>{
      setTimeout(()=>{
        console.log('setTimeout 2');
        clearInterval(interval);
      },0);
    })
},0);

Promise.resolve()
  .then(()=>{
     console.log('promise2');
   });console.log('start');

const i