对这个代码有点困惑，求助！

<html>
    <head>
            <title></title>
                        <script language="javascript">
                            var i=600;
                                 function f()
                                 {var imin,isec;
                                   if(i>0)
                                     { isec=i%60;                                                                                imin=(i-isec)/60;
                                                                        adv.innerText=imin+"分"+isec+"秒";
                                                      }
                                          else
                                         adv.innerText="考试结束";
                                                 i--;
                                                 if(i<0)
                                                  {
                                                     i=0;
                                                  }                                                     window.setTimeout("f()",10);
                                 }
                        </script>   
    </head>
    <body>
         <span name="adv" id="adv" onClick="f()">点击此处开始倒计时</span>
  </body>
</html> 
这个程序当函数第二次调用时，i=599;然后和60取余，在这里程序就应该出现错误啊，因为取余是个无限循环啊，但这个程序运行起来没错，可以进行倒计时，还有就是if(i<0){i=0}这个代码有必要写吗？求高手解答，如果需要，请加QQ362301209，说明是JS朋友