手写 AJAX

const ajax = (method, url, data, success, fail) => {
    var 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()
}