Linted version for SO Fullscreen Answer: https://stackoverflow.com/a/54895933/1774938

fullscreen.js

function fullscreen() {
  const doc = document;
  const elm = doc.documentElement;
  if (elm.requestFullscreen) {
    (!doc.fullscreenElement ? elm.requestFullscreen() : doc.exitFullscreen())
  } else if (elm.mozRequestFullScreen) {
    (!doc.mozFullScreen ? elm.mozRequestFullScreen() : doc.mozCancelFullScreen())
  } else if (elm.msRequestFullscreen) {
    (!doc.msFullscreenElement ? elm.msRequestFullscreen() : doc.msExitFullscreen())
  } else if (elm.webkitRequestFullscreen) {
    (!doc.webkitIsFullscreen ? elm.webkitRequestFullscreen() : doc.webkitCancelFullscreen())
  } else {
    console.log('Fullscreen support not detected.');
  }
}