So thanks to @Mukeyii I found the answer (for a Mac and an iPhone at least) on this page
Here are the steps :
- On your Node app, you need to give the port and the ip you'll be listening the app, for example :
const app = require('./app'); // app is made with express.js
app.set('port', 80);
const server = app.listen(app.get('port'), '0.0.0.0', () => {
  console.log(`Express running → PORT ${server.address().port}`);
});
Strangely, I wanted to put the 127.0.0.1 ip but it didn't work. 0.0.0.0 worked for me so I put that one.
- On the - /private/etc/hostsfile, add the line- 0.0.0.0    any.url.you.wish, save and test on a browser on your computer that when you start your app (dev mode) and you type the url- any.url.you.wish, you go on your app.
 
- Download SquidMan and open it, let it install what it wants to install. 
- In SquidMan > Preferences, set the port in the General tab (for me: 80) 
- On the Client tab, set a new ip: the ip address of the iOS mobile device. To get it, you need to click on the - iin front of your WiFi connected network.
 
- On the Template tab, add a comment on the line - http_access deny to_localhostand add these two lines :
 
# hosts file
hosts_file /private/etc/hosts
- Save the Preferences and Start Squid
- Get the IP Address of the Mac in System Preferences > Network
- Configure the Proxy on the iPhone (port + ip) at the bottom of the same view described in 5.
That's it ! Read more infos in this url: http://egalo.com/2012/05/29/testing-mac-web-site-using-local-hostname-on-mobile-device/