因为写的时候用的let 参考

 var  msIe = false;
      var  ua = window.navigator.userAgent;
      var  old_ie = ua.indexOf('MSIE ');
      var  new_ie = ua.indexOf('Trident/');

      if ((old_ie > -1) || (new_ie > -1)) {
        msIe = true;
      }

      // ie浏览器引用js文件
      if ( msIe ) {
        document.getElementById("polyfillSrc").src = "https://cdn.polyfill.io/v2/polyfill.min.js";
      }