code: function updateEvent(value) {
    switch (typeof value) {
      case 'function': return 'return (' + escape(
        JS_SHORTCUT.test(value) && !JS_FUNCTION.test(value) ?
          ('function ' + value) :
          value
      ) + ').call(this, event)';
      case 'object': return '';
      default: return escape(value || '');
    }
  }
docs: return the right callback to invoke an event stringifying the callback and invoking it to simulate a proper DOM behavior