custom-endpoints.php

class MAP_API_DemoItemType extends WP_JSON_CustomPostType {
	protected $base = '/map/demo';
	protected $type = 'demo';

	public function register_routes( $routes ) {
		$routes = parent::register_routes( $routes );
		return $routes;
	}
}

function map_api_init( $server ) {
	global $map_api_demoitemtype;
	$map_api_demoitemtype = new MAP_API_DemoItemType( $server );
	add_filter( 'json_endpoints', array( $map_api_demoitemtype, 'register_routes' ) );
}
add_action( 'wp_json_server_before_serve', 'map_api_init' );