定时器的启动和停止

<script>
        window.onload=function () {
            var count=document.getElementById("num");
            var num=1;
          var time= setInterval(function () {
                count.innerHTML=num++
                //关闭定时器
                if(num==50){
                    clearInterval(time);
                }
            },100);
        };

    </script>