倒计时实现从10到0，每秒减1

<div>
      <h2> 剩余时间： <span class="clock">10</span></h2>
</div>

    <script type="text/javascript">
            var number = 10;
            var clock = document.getElementsByClassName('clock')[0];
            function clock_time (){
                number --;
                clock.innerHTML = number;
                if(number <= 0){
                 clearInterval(time);
                }
                
            }
            var time = setInterval('clock_time()',1000)
        
    </script>