Create a Woocommerce product sold in units of kilogram and gram

// Add a product custom field "grams_quantity" that will update the displayed price
add_action('woocommerce_before_add_to_cart_button', 'special_product_by_grams', 25);
function special_product_by_grams(){
    global $product;

    // HERE Define the special product IDs sold by grams
    $targeted_product_ids = array(37);

    // or HERE Define a product categories (ids, slugs or names)
    $categories = array('sold-by-gram');

    // Only for products sold by gram
    $product_id = $product->get_id();
    if ( ! ( in_array( $product_id, $targeted_product_ids ) || has_term( $categories, 'product_cat', $product_id ) ) ) return;
    ?>
        <div class="grams-field">
            <label for="grams_quantity"><?php _e('Kilograms: ','woocoomerce'); ?>&nbsp;&nbsp;&nbsp;&nbsp;<?php _e('Grams: ','woocoomerce'); ?><span></span><br>
                <select name="kg_quantity" class="kg_quantity" id="kg_quantity">
                <option value="0">Select Kilogram</option>
                <option value="1000" selected>1 Kg</option>
                <option value="2000">2 Kg</option>
                <option value="3000">3 Kg</option>
                <option value="4000">4 Kg</option>
                <option value="5000">5 Kg</option>
                <option value="10000">10 Kg</option>
                <option value="20000">20 Kg</option>
                <option value="50000">50 Kg</option>
                </select>
                <select name="grams_quantity" class="grams_quantity" id="grams_quantity">
                <option value="0">Select Gram</option>
                <option value="100">100 Gm</option>
                <option value="200">200 Gm</option>
                <option value="250">250 Gm</option>
                <option value="500">500 Gm</option>
                </select>
                <!--<input type="number" step="1" name="grams_quantity" class="grams_quantity" id="grams_quantity" value="1">-->
            </label>
        </div><br>

        <script type="text/javascript">
        function cal_price(priceByGram){
            updatedPrice = ((( parseInt(jQuery('#kg_quantity').val())+parseInt(jQuery('#grams_quantity').val())) * priceByGram)/1000).toFixed(2);
            return updatedPrice;
        }
            (function($){
                // variables initialization
                var priceByGram = <?php echo wc_get_price_to_display( $product ); ?>,
                    currencySymbol = $(".woocommerce-Price-currencySymbol").html(),
                    updatedPrice;

                // On live event: imput number fields
                $('select#kg_quantity').on( "change", function(){
                    updatedPrice = cal_price(priceByGram);
                    $(".woocommerce-Price-amount.amount").html('<span class="woocommerce-Price-amount amount">'+updatedPrice+'&nbsp;'+currencySymbol+'</span>');
                    console.log("event"); // <== To be removed
                });
                $('select#grams_quantity').on( "change", function(){
                    updatedPrice = cal_price(priceByGram);
                    $(".woocommerce-Price-amount.amount").html('<span class="woocommerce-Price-amount amount">'+updatedPrice+'&nbsp;'+currencySymbol+'</span>');
                    console.log("event"); // <== To be removed
                });
            })(jQuery);
        </script>
    <?php
}

// Save the "grams_quantity" custom product field data in Cart item
add_filter( 'woocommerce_add_cart_item_data', 'save_in_cart_the_custom_product_field', 10, 2 );
function save_in_cart_the_custom_product_field( $cart_item_data, $product_id ) {
    if( isset( $_POST['grams_quantity'] ) ) {
        $cart_item_data[ 'grams_quantity' ] = $_POST['grams_quantity'];

        // When add to cart action make an unique line item
        $cart_item_data['unique_key'] = md5( microtime().rand() );
        WC()->session->set( 'custom_data', $_POST['grams_quantity'] );
    }
    return $cart_item_data;
}

// Update product price by grams in cart and checkout
add_filter( 'woocommerce_before_calculate_totals', 'update_prices_by_gram', 10, 1 );
function update_prices_by_gram( $cart_object ) {

    // HERE Define the special product IDs sold by grams
    $targeted_product_ids = array(37);

    // or HERE Define a product categories (ids, slugs or names)
    $categories = array('sold-by-gram');

    foreach ( $cart_object->get_cart() as $cart_item ) {

        // Only for products sold by gram
        $product_id = $cart_item['product_id'];
        if ( in_array( $product_id, $targeted_product_ids ) || has_term( $categories, 'product_cat', $product_id ) ){

            // Get an instance of the WC_Product object and the
            $product = $cart_item['data'];

            $grams = $cart_item['grams_quantity'];

            // Method is_on_sale() manage everything (dates…)
            $product->set_price( $product->get_price() * $grams);
        }
    }
}

// Render "grams_quantity" the custom product field in cart and checkout
add_filter( 'woocommerce_get_item_data', 'render_product_custom_field_meta_on_cart_and_checkout', 10, 2 );
function render_product_custom_field_meta_on_cart_and_checkout( $cart_data, $cart_item ) {
    $custom_items = array();

    if( !empty( $cart_data ) )
        $custom_items = $cart_data;

    if( isset( $cart_item['grams_quantity'] ) )
        $custom_items[] = array(
            'name'      => __( 'Grams', 'woocommerce' ),
            'value'     => sanitize_text_field( $cart_item['grams_quantity'] ),
            'display'   => sanitize_text_field( $cart_item['grams_quantity'] ),
        );

    return $custom_items;
}

// Save "grams_quantity" to the order items meta data
add_action('woocommerce_add_order_item_meta','add_product_custom_fiel_to_order_item_meta', 1, 3 );
function add_product_custom_fiel_to_order_item_meta( $item_id, $item_values, $item_key ) {

    if( isset( $item_values['grams_quantity'] ) )
        wc_update_order_item_meta( $item_id, 'Grams', sanitize_text_field( $item_values['grams_quantity'] ) );
}