JS一个简单的计时器

带例子：以防以后还得在查找

<script type="text/javascript"> 
    var countdown=60; 
    function sendemail(){
        var obj = $("#btn");
        var mobile = $("#textfield").val();
        if(obj.html()=='获取验证码' || obj.html()=='重新发送' ){
            if (!/^1[3|4|5|7|8][0-9]\d{8}$/.test(mobile)) {
                //$('.error').html('手机号码不正确');
                layer.msg('手机号码不正确');
                return false;
            }
            $.ajax({
                type: 'POST',
                url: '/user/regBindSendMsg',
                data: {
                    username: mobile,
                },
                dataType: 'json',
                success: function (msg) {
                    if (msg.code == '200') {
                        //成功调用发送倒计时函数
                        settime(obj);                                           
                    } else {
                        //$('.error').html(msg.info);
                        layer.msg(msg.info);
                    }
                }
            })
            
        }
    }
    function settime(obj) { //发送验证码倒计时
        
        if (countdown == 0) {
            obj.html("重新发送");
            countdown = 60; 
            return;
        } else {
            obj.html(  countdown );
            countdown--; 
        } 
        setTimeout(function() { 
            settime(obj) }
        ,1000)
    }
</script>