事件执行队列

例题分析
例题摘自：总结事件轮询机制，以及宏任务队列与微任务队列

console.log('======== main task start ========');
new Promise(resolve => {
  console.log('create micro task 1');
  resolve();
}).then(() => {
  console.log('micro task 1 callback');
  setTimeout(() => {
    console.log('macro task 3 callback');
  }, 0);
})

console.log('create macro task 2');
setTimeout(() => {
  console.log('macro task 2 callback');
  new Promise(resolve => {
    console.log('create micro task 3');
    resolve();
  }).then(() => {
    console.log('micro task 3 callback');
  })
  console.log('create macro task 4');
  setTimeout(() => {
    console.log('macro task 4 callback');
  }, 0);
}, 0);

new Promise(resolve => {
  console.log('create micro task 2');
  resolve();
}).then(() => {
  console.log('micro task 2 callback');
})

console.log('======== main task end ========');