how to add custom javascript in wordpress child theme

Here is best way to include custom js in child theme:

function my_scripts_method() {
wp_enqueue_script(
    'custom-script',
    get_stylesheet_directory_uri() . '/js/custom_script.js',
    array( 'jquery' )
 );
}

add_action( 'wp_enqueue_scripts', 'my_scripts_method' );

You can use the wp_enqueue_script() function.

documentation: https://developer.wordpress.org/reference/functions/wp_enqueue_script/

This function allows you to load scripts at any given time.

Create a folder in child theme and upload your JS file to that folder.

You can include a js file using

wp_enqueue_script()

or you can use

<script src="<?php echo get_stylesheet_directory_uri(); ?>/js/filename.js"  type="text/javascript"></script>

add the above code in theme header.php or footer.php