定时器

var timer = setInterval(function () {
          if (countdown <= 0) {
              clearInterval(timer);
              $("#yzmdm").css('display','none');
              $("#validationCode").css('display','block');
              countdown = 60;
          }
          $("#yzmdm").html('重新发送('+(countdown-1)+'s)');
          countdown--;
      }, 1000);