手写 AJAX

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AJAX:async javaScript and xml,异步 js 和 json

const ajax = (method, url, data, success, fail) => {
  var request = new XMLHttpRequest()
  request.open(method, url);
  request.onreadystatechange = function () {
    if (request.readyState === 4) { // 表示下载完成
      if (request.status >= 200 && request.status < 300 || request.status === 304) {
      // 表示成功的状态码,304表示内容不变,所以也算成功
        success(request)
      } else {
        fail(request)
      }
    }
  };
  request.send();
}