Enqueue script

<?php

/**
 * theme functions and definitions
 *
 * @link https://developer.wordpress.org/themes/basics/theme-functions/
 *
 * @package theme
 */


/**
* Enqueue scripts and styles.
*/
function theme_scripts() {
    wp_enqueue_style( 'theme-style', get_stylesheet_uri(), array(), 0 );
    wp_enqueue_style('google-fonts', '//fonts.googleapis.com/css2?family=Inter:wght@400;600;700;900&family=Roboto+Slab:wght@900&display=swap', array(), 0);
    
    //We have added this line of code
    wp_enqueue_script( 'default-js', get_template_directory_uri() . '/app.js', array(), 0, true );

}
add_action( 'wp_enqueue_scripts', 'theme_scripts' );