js实现几秒后页面自动跳转

<script type="text/javascript">
var num = 5;
function countDown() {
if (num >= 0) {
var str = '';
str += '系统将在 ' + num + ' 秒钟后自动返回 <a href="http://">首页<\/a>';
document.getElementById('autoDir').innerHTML = str;
num--;
setTimeout(countDown, 1000);
}
else {
window.location.href = "${cms.AppPath}/HealthIndex.aspx";
}
}
window.onload = countDown;
</script>

<div id="autoDir"></div>