Source

File: wp-includes/class-wp-customize-nav-menus.php

public function export_preview_data() {

		// Why not wp_localize_script? Because we're not localizing, and it forces values into strings.
		$exports = array(
			'navMenuInstanceArgs' => $this->preview_nav_menu_instance_args,
		);
		printf( '<script>var _wpCustomizePreviewNavMenusExports = %s;</script>', wp_json_encode( $exports ) );
	}