js はブラウザが PC かモバイルかをどのように判断するのでしょうか? (2つの方法)

<script type="text/javascript">
var mobileAgent = new Array("iphone", "ipod", "ipad", "android", "mobile", "blackberry", "webos", "incognito", "webmate", "bada", "nokia", "lg", "ucweb", "skyfire");
var browser = navigator.userAgent.toLowerCase();
var isMobile = false;
for (var i = 0; i < mobileAgent.length; i++) {
if (browser.indexOf(mobileAgent[i]) != -1)
 {
isMobile = true;//alert(mobileAgent[i]);
location.href = &#39;手机要访问页面的链接&#39;;
break;
}
}
</script>

if(/Android|webOS|iPhone|iPod|BlackBerry/i.test(navigator.userAgent))
 {
    window.location.href = "https://www.baidu.com/";
    } else {
    window.location.href = "http://news.baidu.com/";
    }