相关代码：

       // 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('?')){
            if(url.substr(url.length-1,1)=='?'||url.substr(url.length-1,1)=='&'){
                url+=""
            }else{
                url+='&'
            }
         }else{
             url += '?'
         }

         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>');