js协议限时效果

 <script type="text/javascript">

        window.onload = function () {

            var sec = 5;
            var intervalId = setInterval(function () {
                sec--;
                var btnobj = document.getElementById('btn');
                if (sec > 0) {
                    btnobj.value = '请仔细阅读协议(' + sec + ')';
                } else {
                    //设置按钮上显示的文字为同意.
                    btnobj.value = '同意';
                    btnobj.disabled = false;
                    clearInterval(intervalId);

                }
            }, 1000);


        };
    </script>