AJAX：异步的JavaScript和XML
const request = new XMLHttpRequest()
request.open("GET", url)
request.onreadystatechange = () =>{
if(request.readyState === 4){
console.log("请求完成")
if(request.status >= 200 && request.status < 300){
console.log("请求成功")
}
}
}
request.send()
