Price calculation based on dimensions in WooCommer

Based on "Get selected variation price in jQuery on Woocommerce Variable products" answer code,
in my code bellow, I have a problem with the price calculation of a WooCommerce variable product.

The price gets multiplied with 10 or 1000, depending on the option selected on a dropdown, which is not supposed to happen and I don't know why it is happening.

Here is my code:

<script>
    jQuery(function($) {
        var jsonData = <?php echo json_encode($variations_data); ?>,
            inputVID = 'input.variation_id';

        $('input').change( function(){
            if( '' != $(inputVID).val() ) {
                var vid      = $(inputVID).val(), // VARIATION ID
                    length   = $('#cfwc-title-field').val(), // LENGTH
                    diameter = $('#diameter').val(),  // DIAMETER
                    ene_enden = $('#id_dropdown_one_end').find('option:selected').attr("value_one_end"),
                    vprice   = ''; // Initilizing

                // Loop through variation IDs / Prices pairs
                $.each( jsonData, function( index, price ) {
                    if( index == $(inputVID).val() ) {
                        vprice = price; // The right variation price
                    }
                });
                var rope_price = (length*vprice) + ene_enden;
                if (rope_price != 0){
                    $('.price').html(rope_price+',-');
                }

                alert('variation Id: '+vid+' || Lengde: '+length+' || Diameter: '+diameter+' || Variantpris: '+vprice+ ' || Rope price: '+rope_price+' || ene_enden = '+ene_enden);

            }
        });
    });
</script>

For some reason rope_price gets multiplied by 10 or concatenated with 0 when the option selected for 'I enden av tauet ' is 'Ingenting'(it's value is 0). When I change the option selected to any of the others rope_price gets multiplied with 1000 or concatenated with 00. I don't know why this is happening. Any ideas?

Because you are concatenating strings. Is not the same 1 + 0 than "1" + "0", as you can check here:

console.log("1 + 0 =", 1 + 0);
console.log('"1" + "0" =', "1" + "0");

When you get a value from an HTML object, you receive it as a string. If you want to use it as a number you must convert it before. You can use either Number or parseFloat (even parseInt, but will remove decimals).

var oneNumber = 1;
var oneString = "1";
var oneConverted = Number(oneString);

console.log("typeof oneNumber:", typeof oneNumber);
console.log("typeof oneString:", typeof oneString);
console.log("typeof oneConverted:", typeof oneConverted);

console.log("oneNumber + oneNumber =", oneNumber + oneNumber);
console.log('oneString + oneString =', oneString + oneString);
console.log('oneConverted + oneConverted =', oneConverted + oneConverted);

The exact problem you are having is your ene_enden variable being a string in the line var rope_price = (length*vprice) + ene_enden;. When you multiply two strings, they are automatically converted to a number (your (length*vprice)), but when you concatenate that number to another string, they are converted automatically to a string again (your + ene_enden), so you must first convert ene_enden to a number, ot better convert all expected number variables to a number.