Doubt - Site for Mobile Device

If it is Node.js with Express (so I noticed it looks like it's what you use) or it's on the front end you can then use

• link

Node.js / Express:

First install via npm :

npm install mobile-detect --save

Then in your project do something like this:

var MobileDetect = require('mobile-detect');
var md = new MobileDetect(req.headers['user-agent']);

if (md.mobile()) {
    res.redirect('http://mobile.site.com');
} else if (String(req.get('host')).indexOf('mobile.') == 0) {
    //Acaso acessar o site mobile no navegador Desktop ele irá redirecionar para o site normal
    res.redirect('http://www.site.com');
}

Browser / front-end

If it is to be used in the browser, you can either download mobile-detect.min.js or use CDN, like this:

<script src="//cdnjs.cloudflare.com/ajax/libs/mobile-detect/1.4.1/mobile-detect.min.js"></script>

It should look like this:

<script src="mobile-detect.min.js"></script>
<script>
var md = new MobileDetect(window.navigator.userAgent);

if (md.mobile()) {
    window.location.replace('http://mobile.site.com');
} else if (window.location.hostname.indexOf('mobile.') == 0) {
    //Acaso acessar o site mobile no navegador Desktop ele irá redirecionar para o site normal
    window.location.replace('http://www.site.com');
}
</script>