Return JSON data from action of Joomla controller.

joomla_json.php

/**
 * JSON response from controller's action.
 */
public function run( )
{
    JFactory::getDocument()->setMimeEncoding( 'application/json' );
    JResponse::setHeader('Content-Disposition','attachment;filename="progress-report-results.json"');

    $data = array(
        'foo' => 'bar'
    );
    echo json_encode( $data );
    JFactory::getApplication()->close(); // or jexit();
}