How to add javascript code into Divi child theme?

You’re almost there, change code in functions.php to:

<?php
function my_theme_enqueue_styles() {

    $parent_style="divi-style";

    wp_enqueue_style( $parent_style, get_template_directory_uri() . '/style.css' );
    wp_enqueue_style( 'child-style',
        get_stylesheet_directory_uri() . '/style.css',
        array( $parent_style ),
        wp_get_theme()->get('Version')
    );
    wp_enqueue_script( 'et_mobile_nav_menu', get_stylesheet_directory_uri() . '/js/selectpage.js', array( 'jquery' ), true );
}
add_action( 'wp_enqueue_scripts', 'my_theme_enqueue_styles' );
?>

Now your script should be located at: /wp-content/themes/divi-child/js/selectpage.js