///////////////////////
        // 定义ajax-promise.js
        ///////////////////////

        var AjaxPromise = function(url, method, header, data) {
          return new Promise(function(resolve, reject) {
            var xhr = createXHR();
            xhr.open(method, url, true);
            for (var key in header) {
              xhr.setRequestHeader(key, header[key]);
            }
            xhr.onreadystatechange = function() {
              if (xhr.readyState == 4) {
                if (xhr.status == 200) {
                  resolve(xhr.responseText);
                } else {
                  reject('请求失败');
                }
              }
            };
      
            xhr.send(data);
          });
        };


        ///////////////////////
        // 使用
        ///////////////////////

        <script src="../promise/libs/promise.js"></script>
        <script src="libs/ajax-promise.js"></script>

        AjaxPromise('https://cn.vuejs.org/images/logo.png', 'GET', {}, {})
          .then(function (res) {

            console.log(res);

          }).catch(function (err) {

            console.log(err);
            
          });