HTML5 FullScreen API toggle with JavaScript

function goFullScreen() {
  const el = document.documentElement,
      rfs = el.requestFullScreen
        || el.webkitRequestFullScreen
        || el.mozRequestFullScreen
        || el.msRequestFullscreen

  rfs.call(el)
}

document.querySelector('#full-screen-button')
  .addEventListener('click', () => {
    goFullScreen()
  })

Keep in mind that requesting fullScreen needs to be done via a user-triggered event such as a click event - mousedown,mouseup etc..

Changing the 1st line of _isFullScreen function to

fullScreen = document.fullscreenEnabled || document.mozFullscreenEnabled || document.webkitFullscreenEnabled ? true : false;

Does the trick (at least for Firefox, Chrome and Safari on Mac and Windows)