Calculation of percentage in javascript

Question

Hello!

I’m with a question of those about a discount calculation code in javascript, below follows the code

JS

function ValorMensal () {
    var mensalidade, desconto, descontofinal, precofinal;
    mensalidade = parseFloat(document.getElementById("CAMPO_27").value);
    desconto = parseFloat(document.getElementById("CAMPO_25").value);
    descontofinal = mensalidade * desconto / 100;
    precofinal = mensalidade - descontofinal;
    document.getElementById("CAMPO_18").value = precofinal;
}

What happens, I have 2 problems, I’m almost 3 weeks and I can’t solve

When I play only the value of the monthly fee field, it should theoretically appear in the monthly and final field, regardless of whether there is a discount, but to my surprise it appears exactly the same as the image below

And my other question is about the question of "," and "."

When I calculate discount with, for example. 2314.50, it rounds and when I put 2314.50, it gives me the exact amount. Can anyone give me a light? From now on I’m beaten