Safari 10.1 `nomodule` support

safari-nomodule.js

/**
 * Safari 10.1 supports modules, but does not support the `nomodule` attribute - it will
 * load <script nomodule> anyway. This snippet solve this problem, but only for script
 * tags that load external code, e.g.: <script nomodule src="nomodule.js"></script>
 *
 * Again: this will **not** prevent inline script, e.g.:
 * <script nomodule>alert('no modules');</script>.
 *
 * This workaround is possible because Safari supports the non-standard 'beforeload' event.
 * This allows us to trap the module and nomodule load.
 *
 * Note also that `nomodule` is supported in later versions of Safari - it's just 10.1 that
 * omits this attribute.
 */
(function() {
  var check = document.createElement('script');
  if (!('noModule' in check) && 'onbeforeload' in check) {
    var supportModule = false;
    document.addEventListener('beforeload', function(e) {
      if (e.target === check) {
        supportModule = true;
      } else if (e.target.hasAttribute('nomodule') && supportModule) {
        e.preventDefault();
      }
    }, true);

    check.type = 'module';
    check.src = '.';
    document.head.appendChild(check);
    check.remove();
  }
}());