Issue with Variation

Hello,
I am using the following code to add a Buy Now button on my website. The code is by LoicTheAztec

Here is the code:

add_action( 'woocommerce_after_add_to_cart_button', 'add_custom_addtocart_and_checkout' );
function add_custom_addtocart_and_checkout() {
    global $product;

    $addtocart_url = wc_get_checkout_url().'?add-to-cart='.$product->get_id();
    $button_class  = 'single_add_to_cart_button button alt custom-checkout-btn';
    $button_text   = __("Buy & Checkout", "woocommerce");

    if( $product->is_type( 'simple' )) :
    ?>
    <script>
    jQuery(function($) {
        var url    = '<?php echo $addtocart_url; ?>',
            qty    = 'input.qty',
            button = 'a.custom-checkout-btn';

        // On input/change quantity event
        $(qty).on('input change', function() {
            $(button).attr('href', url + '&quantity=' + $(this).val() );
        });
    });
    </script>
    <?php

    elseif( $product->is_type( 'variable' ) ) : 

    $addtocart_url = wc_get_checkout_url().'?add-to-cart=';
    ?>
    <script>
    jQuery(function($) {
        var url    = '<?php echo $addtocart_url; ?>',
            vid    = 'input[name="variation_id"]',
            pid    = 'input[name="product_id"]',
            qty    = 'input.qty',
            button = 'a.custom-checkout-btn';

        // Once DOM is loaded
        setTimeout( function(){
            if( $(vid).val() != '' ){
                $(button).attr('href', url + $(vid).val() + '&quantity=' + $(qty).val() );
            }
        }, 300 );

        // On input/change quantity event
        $(qty).on('input change', function() {
            if( $(vid).val() != '' ){
                $(button).attr('href', url + $(vid).val() + '&quantity=' + $(this).val() );
            }
        });

        // On select attribute field change event
        $('.variations_form').on('change blur', 'table.variations select', function() {
            if( $(vid).val() != '' ){
                $(button).attr('href', url + $(vid).val() + '&quantity=' + $(qty).val() );
            }
        });
    });
    </script>
    <?php
    endif;
    echo '<a href="'.$addtocart_url.'" class="'.$button_class.'">'.$button_text.'</a>';
}

Now, the problem is whenever I am clicking on the Buy Now button with the default variations set in the Product editor, the code is not working.

The Buy Now URL in this case becomes …/?addtocart=0&quantity=n

Any Custom JS that might do the job?

P.S: Tried with a lot of themes, the code works well with all of them.

Thanks