一直在用fetch进行网络请求，我都知道fetch是支持Promise的，突发奇想想要自己实现一个简易版的基于Promise的的ajax；

const ajax = (method, url, data) => {
      return new Promise((resolve, reject) => {
        const xhr = new XMLHttpRequest();
        const upperMethod = method.toUpperCase();
        xhr.open(upperMethod, url, true);
        if (upperMethod === 'POST') {
          xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
          xhr.send(JSON.stringify(data));
        } else {
          xhr.send();
        }
        xhr.onload = resolve;
        xhr.onerror = reject;
      })
    }