How to add a custom.js file for a child-theme in a swift way?

That worked for me:

wp_enqueue_script( 
    'YOUR_SCRIPT_NAME',
    get_stylesheet_directory_uri() . '/YOUR_SCRIPT_FILE.js',
    array(),
    '4.7.3',
    true 
);

Notes:

1. I usually name the script just as its file name.
2. You don’t have to use the version number 4.7.3, but you do need to write some plausible version number (like of your WordPress core version) or the keyword null instead.