[javascript] Detecting a mobile browser

Depends on the use case. All mobile devices require a battery. If what you're after is compute power without draining the battery use the Battery Status API:

navigator.getBattery().then(battery => {
  battery.charging ? 'charging' : 'not charging';
});

If what you're looking for is presentational use matchMedia, which returns a Boolean value:

if (window.matchMedia("(min-width: 400px)").matches) {
  /* the viewport is at least 400 pixels wide */
} else {
  /* the viewport is less than 400 pixels wide */
}

Or combine them for an even better user experience on tablet devices.