If you want to detect with javascript if a user is using a mobile device, here is one way you can do it:

const agent = (device) => navigator.userAgent.indexOf(device) > -1;

const isDevice = {
  iphone: () => agent('iPhone'),
  ipod: () => agent('iPod'),
  ipad: () => agent('iPad'),
  android: () => agent('Android'),
  blackberry: () => agent('BlackBerry'),
  opera: () => agent('Opera Mini'),
  windows: () => agent('IEMobile'),
};

if(isDevice.iphone()) {
  console.log('Iphone device detected');
  // Iphone device detected
}