Enqueue js script to footer

Have you looked at wp_enqueue_script in the WordPress codex? You’ll most likely need to add this to functions.php in your theme’s directory.

Also, where are you enqueuing the startapp_js dependency?

function prefix_enqueue_scripts() {
    wp_enqueue_script( 'startapp-js', get_template_directory_uri() . '/scripts/Startapp.js', array(), '', true ); 
    wp_enqueue_script( 'app-js', get_template_directory_uri() . '/scripts/App.js', array( 'startapp-js' ), '', true );
}
add_action( 'wp_enqueue_scripts', 'prefix_enqueue_scripts' );