jsonp的promise封装

/*
    @url 请求地址
    @超时时间
    @return promise
*/
var getJSONP =  function(url, overtime) {
  var promise = new Promise(function(resolve, reject) {
    //注册回调函数
    var callbackName = 'jsonp'+Math.ceil(Math.random() * 1000000);
      window[callbackName] = function(json) {
      head.removeChild(script);    //移除scipt标签
      clearTimeout(script.timer);  //清除超时计时器
      window[callbackName] = null;
      resolve(json);   //成功处理
    }
    //生成scipt标签
    var script = document.createElement('script');
    script.src = url;  
    var head = document.getElementsByTagName('head')[0];
    head.appendChild(script);
    //错误处理
    script.onerror = function(){  
        if(window[callbackName] == null){  
            reject('timeout');   //超时处理
        }  else{
            reject('faild');   //失败处理
            window[callbackName] = null;
          head.removeChild(script);
        }
    }  
    //超时判断
    script.timer = setTimeout(function () {
        window[callbackName] = null;
        head.removeChild(script);
    }, overtime);
  });
  return promise;
};