<script>

      // 3.应用
      let url = 'https://www.imooc.com/course/list?';
      let url1 = 'https://www.imooc.com/course/list?username=xm';
      let url2 = 'https://www.imooc.com/course/list?username=xm&';

      const addURLParam = (url, name, value) => {
         // 在此补充代码
         if (url.includes('?')) {
            const str = url.substr(url.length - 1, 1);
            if (str == '?' || str == '&') {
               url += '';
            } else {
               url += `&`;
            }
         } else {
            url += `?`;
         }
         url += url.includes('?') ? '&' : '?';
         url += `${name}=${value}`;
         return url;
      };
      url = addURLParam(url, 'password', 'admin');
      url1 = addURLParam(url1, 'password', 'admin');
      url2 = addURLParam(url2, 'password', 'admin');
      document.write(url, '<br>');
      document.write(url1, '<br>');
      document.write(url2, '<br>');
   </script>

​