Add a action to enqueue stylesGist for my "WorpdPress: Css Styles and Javascripts in theme development (In depth)" article: https://since1979.dev/wordpress-css-styles-and-javascripts-in-theme-development-in-depth/

functions-styles-scripts-add-script.php

<?php
function my_theme_enqueue_scripts()
{
  /* Add a local javascript file */
  wp_enqueue_script( 'my-theme-scripts', get_template_directory_uri() . '/public/js/app.js', array(), '1.0.0', true );
}

add_action( 'wp_enqueue_scripts', 'my_theme_enqueue_scripts' );
?>