Adding a script in WordPress, with IE conditional.

functions.php

add_action( 'wp_enqueue_scripts', 'cs_conditional_scripts_and_styles' );
/**
 * Enqueue styles and scripts conditionally.
 *
 * @link https://msdn.microsoft.com/en-us/library/ms537512(v=vs.85).aspx
 * @since 1.0.0
 */
function cs_conditional_scripts_and_styles() {

	/**
	 * Enqueue flexiility.js script for users of IE9 and below.
	 * @link: https://github.com/10up/flexibility
	 */
	wp_enqueue_script( 'flexibility', get_stylesheet_directory_uri() . '/assets/js/flexibility.js', [], '20160502', true );
	wp_script_add_data( 'flexibility', 'conditional', 'lte IE 9' );

}