手写 AJAX

const ajax = (method, url, data, success, fail)=>{
    const request = new XMLHttpRequest()
    request.open(method, url)
    request.onreadystatechange =function (){
        if(request.readyState === 4){
            if(request.status >= 200 && request.status < 300 || request.status === 304){
                success(request)
            }else{
                fail(request)
            }
        }
    }
    request.send()
}