if(/AppleWebKit.*Mobile/i.test(navigator.userAgent) ||
   /MIDP|SymbianOS|NOKIA|SAMSUNG|LG|NEC|TCL|Alcatel|BIRD|DBTEL|Dopod|PHILIPS|HAIER|LENOVO|MOT-|Nokia|SonyEricsson|SIE-|Amoi|ZTE/.test(navigator.userAgent)) {
  if(location.href.indexOf('?mobile') < 0) {
    if(/Android|webOS|iPhone|iPod|BlackBerry/i.test(navigator.userAgent)) {
      location.href = '../m/index.html';
    } else if(/iPad/i.test(navigator.userAgent)) {
      // ipad branch
    }
  }
}

function getPlatform(){
  const ua = navigator.userAgent.toLowerCase();
  if(/android/.test(ua)) return 'android';
  if(/iphone|ipad/.test(ua)) return 'ios';
  if(/windows/.test(ua)) return 'wp';
  return 'pc';
}

Modern advice: prefer responsive design + @media queries; UA parsing is brittle.