ExportController & ExportJsonView

ExportController.php

<?php
namespace VENDOR\Export\Controller;

class ExportController
    extends \TYPO3\CMS\Extbase\Mvc\Controller\ActionController
{
    /**
     * A list of formats and object names of the views which should render them
     *
     * @var array
     */
    protected $viewFormatToObjectNameMap = [
        'json' => \VENDOR\Export\View\Export\ExportJsonView::class
    ];

    /**
     * @var \VENDOR\MyExtension\Domain\Repository\CustomerRepository
     * @inject
     */
    protected $customerRepository;

    /**
     * Render the template
     *
     * @return void
     */
    public function indexAction() {}

    /**
     * export action
     */
    public function exportAction() {
        $this->view->setVariablesToRender( ['pages'] );
        $this->view->assign( 'pages', $this->pageRepository->findAll() );
        $this->view->setConfiguration(
            [
                // Pages
                'pages' => [
                    '_descend' => [
                        // Customer
                        'customer' => [
                            '_descendAll' => []
                        ]
                    ]
                ]
            ]
        );
    }
}

ExportJsonView.php

<?php
namespace VENDOR\Export\View\Export;

use TYPO3\CMS\Core\Utility\GeneralUtility;

class ExportJsonView
    extends \TYPO3\CMS\Extbase\Mvc\View\JsonView
{
    /**
     * Export path
     */
    const EXPORT_FOLDER = PATH_typo3conf . 'ext/export/data/';

    /**
     * render function
     */
    public function render() {

        $propertiesToRender = $this->renderArray(); 
        $export = json_encode( $propertiesToRender, JSON_PRETTY_PRINT );

        GeneralUtility::writeFile(
            self::EXPORT_FOLDER . 'data.json',
            $export
        );

    }
}