I am trying to have my on-page subtotal for individual WooCommerce products update on a per-quantity basis. I have found a solution that works perfectly for simple products via the following URL: https://medium.com/@xemoro/how-to-display-the-calculated-subtotal-based-on-quantity-in-woocommerce-single-variable-products-24379410a227.

The simple product code is as follows:

add_action( 'woocommerce_after_add_to_cart_button', 'bbloomer_product_price_recalculate' );
function bbloomer_product_price_recalculate() {
   global $product;
   echo '<div id="subtot" style="display:inline-block;">Total: <span></span></div>';
   $price = $product->get_price();
   $currency = get_woocommerce_currency_symbol();
   wc_enqueue_js( "     
      $('[name=quantity]').on('input change', function() { 
         var qty = $(this).val();
         var price = '" . esc_js( $price ) . "';
         var price_string = (price*qty).toFixed(2);
         $('#subtot > span').html('" . esc_js( $currency ) . "'+price_string);
      }).change();
   " );
}

The above URL also has a solution for variable products, and the code of which is as follows:

add_action( 'woocommerce_after_add_to_cart_button', 'calculate_product_subtotal' );

function calculate_product_subtotal() {
   global $product;

   if ( $product->is_type( 'variable' ) ) {
       $default_attributes = $product->get_default_attributes();
       $variation_id = $product->get_matching_variation( $default_attributes );
       $variation = wc_get_product( $variation_id );
       $price = $variation->get_price();
   } else {
       $price = $product->get_price();
   }

   echo '<div id="subtot" style="display:inline-block;">Total: <span></span></div>';

   $currency = get_woocommerce_currency_symbol();

   wc_enqueue_js( "     
      jQuery('[name=quantity]').on('input change', function() { 
         var qty = jQuery(this).val();
         var price = '" . esc_js( $price ) . "';
         var price_string = (price * qty).toFixed(2);
         jQuery('#subtot > span').html('" . esc_js( $currency ) . "' + price_string);
      }).change();
   " );
}

However, the variable product code gives my website a critical error, which reads as follows:

Deprecated: Function WC_Product::get_matching_variation is deprecated since version 3.0! Use Product data store find_matching_product_variation instead.

When I attempt to update ‘get_matching_variation’ with ‘find_matching_product_variation’, I get another critical error that reads as follows:

Fatal error: Uncaught Error: Call to undefined method WC_Product_Variable::find_matching_product_variation()

Thanks for everyones help.