Script cannot get calculate with element assigned to const

Question

I'm trying to modify a cart of a shopify store. It's using a bundle application, which overwrites the prices in the cart, hence I'm trying to get those overwrited prices and calculate with those, in order to be able to display how much the customers are saving with the discount. My code is like this:

<script>
          const original_price_for_product_range = document.getElementsByClassName("revy-line-item-price__old");
          const a = original_price_for_product_range[0].outerText;
          const c = a.replace("€","");
          const d = c.replace(",",".");
          const old_price_for_product = +d;
          
          const discounted_price_for_product_range = document.getElementsByClassName("revy-line-item-price__new");
          const b = discounted_price_for_product_range[0].outerText;
          const f = b.replace("€","");
          const g = f.replace(",",".");
          const new_price_for_product = +g
</script>

Basically I'm getting the old and new price by their class name and then getting the first element out of that array and converting it to a number, I can calculate with. (outer text to get the first price, then removing the € and then replacing , with . so I can convert the text to a number, which I can do calculations with).

My problem is that after getting the elements by their class name, ALL following process just WON'T run, getting "Uncaught ReferenceError: a is not defined at <anonymous>:1:1" errors.

I was expecting the code to get the array first element and convert it to number given the script's code, but it's not working. Can anyone help out with this?