Toggles fullscreen mode by checking which features are available.

toggleFullscreen.js

/**
 * @license Copyright 2020 - Chris West - MIT Licensed
 * @see https://www.yourjs.com/blog/snippet-toggle-fullscreen/
 * Either toggles the fullscreen view on the document or a specified element or
 * turns the fullscreen view on or off depending on the optional value passed
 * in.
 * @param opt_elem {Element|undefined|null}
 *   The element that should be targeted for fullscreen viewing.
 * @param opt_value {?boolean=}
 *   Optional.  If not given or undefined or null fullscreen will simply be
 *   toggled.  If truthy the element will be set to fullscreen.  If falsy and is
 *   not undefined or null fullscreen will be turned off if it is on.
 * @returns {boolean|undefined}
 *   If possible returns a boolean indicating if upon calling the function, an
 *   element was in fullscreen mode.
 */
function toggleFullscreen(opt_elem, opt_value) {
  var doc = document, done, wasFull;
  opt_elem = opt_elem || doc.documentElement;
  '-frsexit ms-FRsExit moz-FRSCancel webkit-FRsExit'.replace(
    /(\w*)-(f)(r)(s)(\w+)/gi,
    function(_, prefix, f, r, s, exitWord) {
      s = 'ull' + s + 'creen';
      if (!done && doc[prefix + f + s + 'Enabled']) {
        // If in fullscreen mode and opt_value falsy.
        if ((wasFull = !!doc[prefix + f + s + 'Element']) && doc[exitWord = prefix + exitWord + 'F' + s] && !opt_value) {
          doc[done = exitWord]();
        }
        // If not in fullscreen mode and opt_value is truthy or opt_value is
        // undefined or null.
        else if (opt_elem[r = prefix + r + 'equestF' + s] && (opt_value || opt_value == null)) {
          opt_elem[done = r](Element && Element.ALLOW_KEYBOARD_INPUT);
        }
      }
    }
  );
  return wasFull;
}

Can be seen in action here:
https://www.yourjs.com/blog/snippet-toggle-fullscreen/