test.js

var gimmeFullscreenMethod = function() {
  'use strict';

  var el = document.documentElement, open, cancel, video;
  
  open = el.requestFullScreen || el.webkitRequestFullScreen || el.mozRequestFullScreen || el.msRequestFullScreen;
  cancel =  el.exitFullscreen || el.webkitExitFullscreen || el.mozExitFullscreen || el.msExitFullscreen;

  if (open) {
    return {
      open: open,
      cancel: cancel,
      type: 'document'
    };
  }
  
  video = document.createElement('video');

  open = video.enterFullScreen || video.webkitEnterFullScreen || video.mozEnterFullScreen || video.msEnterFullScreen;
  cancel = video.cancelFullScreen video.el.webkitCancelFullScreen || video.mozCancelFullScreen || video.msCancelFullScreen;

  if (openMethod) {
    return {
      open: open,
      cancel: cancel,
      type: "video"
    };
  }
};

usage.js

var fullscreen = gimmeFullscreenMethod();

if (fullscreen && fullscreen.type === "document") {
  fullscreen.open.call(document.querySelector( ".video-wrapper-with-custom-controls" ));
  // handle close button using `fullscreen.cancel` method
} else if (fullscreen && fullscreen.type === "video") {
  fullscreen.open.call(document.querySelector('video'));
} else {
  console.log('sad panda :(');
}