话不多说，代码：

function ajaxPromise(url, method = 'GET', data, dataType='JSON', async = 'true', timeout) {
  let xhr = new XMLHttpRequest();
  return new Promise((resolve, reject) => {
    xhr.open(method, url, async);
    xhr.responseType = dataType;
    xhr.timeout = timeout;
    xhr.onreadystatechange = () => {
      // 简单的请求成功判断
      if (xhr.readyState === 4 && xhr.status === 200) {
        let result = xhr.responseText;
        resolve(result);
      } else {
        reject(new Error());
      }
    }
    xhr.send(data);
    // 错误处理
    xhr.onabort = function() {
      reject(new Error({
        msg: 'abort error',
        xhr: xhr
      }));
    }
    xhr.ontimeout = function() {
      reject({
        msg: 'ajax timeout',
        xhr: xhr
      });
    }
    xhr.onerror = function() {
      reject({
        msg: 'onerror',
        xhr: xhr
      })
    }
  })
}