js实现页面的秒数倒计时

<button name="vcode_mail" class="btn btn-default" type="button" id="vcode_mail-btn" onclick="vcode_mail_post(this)">免费获取验证码</button>

    //发送验证码
    function vcode_mail_post(obj){
        var seconds =121;

        //获取邮箱
        var email = $("input[name=‘email‘]").val();
        //异步变同步
        /*$.ajaxSettings.async = false;*/
        //发送请求
        $.post("{:url(‘index/user/vcode_mail_post‘)}",{email:email,type:1},function(data){

            var message = data.message;
            if(data.ok == ‘1‘){
                alert(message);
                //倒计时
                daojishi(seconds,obj);    
            }else{
                alert(message);

            }
        },‘json‘);
        /*$.ajaxSettings.async = true;*/
    }

    //倒计时
    function daojishi(seconds,obj){

        if (seconds > 1){

                seconds--;

                $(obj).text(seconds+"秒后可重新获取 ").attr("disabled", true);//禁用按钮

                // 定时1秒调用一次

                setTimeout(function(){

                    daojishi(seconds,obj);

                },1000);

            }else{

                $(obj).text("重新获取验证码").attr("disabled", false);//启用按钮

            }        
    }