Flarum\Formatter\Formatter::getJs PHP Method

    public function getJs()
    {
        $configurator = $this->getConfigurator();
        $configurator->enableJavaScript();
        $configurator->javascript->exportMethods = ['preview'];
        return $configurator->finalize(['returnParser' => false, 'returnRenderer' => false])['js'];
    }

 /**
  * {@inheritdoc}
  */
 protected function getAssets()
 {
     $assets = parent::getAssets();
     $assets->addJs(function () {
         return $this->formatter->getJs();
     });
     return $assets;
 }