javascript 宏任务与微任务

1.由宿主环境发起的称为宏观任务
2.由javascript引擎发起的称为微观任务

事件循环

宏观任务有：

	浏览器	node
setTimeout	✔️	✔️
setInterval	✔️	✔️
setImmediate	✘	✔️
requestAnimationFrame	✔️	✘

微观任务有：

	浏览器	node
process.nextTick	✘	✔️
MutationObserver	✔️	✘
Promise	✔️	✔️

setTimeout(()=>console.log(1));

const promise = new Promise((resolve, reject) => {
  resolve()
  console.log(2)
})

promise.then(() => {
  console.log(3)
})

console.log(4);
//执行结果
//2
//4
//3
//1