Question

I am having a bit of trouble with a few math calculation in javascript.
The goal of this calculation is to generate a value when the user clicks on a text field.

For example:
1 Kilogram costs 32 cents to ship to America and the user wants to find out what 10KG will cost him which is $3.20. For this I have the following piece of javascript code:

    function calculate(num) {
        var weight  = document.getElementById('weight'+num);
        var price   = document.getElementById('price'+num);
        if(num == undefined || num == '' || num.length <= 0 || isNaN(weight.value) || isNaN(price.value)) return false;

        if(num == 1) multiplyBy = 0.32;
        if(num == 2) multiplyBy = 0.14;
        if(num == 3) multiplyBy = 0.24;
        if(num == 4) multiplyBy = 0.53;

        var sum = parseInt(document.getElementById('weight'+num).value) * multiplyBy;
        if(isNaN(sum)) return false;
        price.value = sum;
    }

The above code works perfectly fine, however when I reverse the process (someone has $3.20 and wants to find out how much KG he/she can ship with that (which is 10KG) the script returns: 9.375KG

The following code is used for this calculation:

function reverse(num) {
        var weight  = document.getElementById('weight'+num);
        var price   = document.getElementById('price'+num);
        if(num == undefined || num == '' || num.length <= 0 || isNaN(weight.value) || isNaN(price.value)) return false;

        if(num == 1) divideBy = 0.32;
        if(num == 2) divideBy = 0.14;
        if(num == 3) divideBy = 0.24;
        if(num == 4) divideBy = 0.53;

        var sum = parseInt(document.getElementById('price'+num).value) / divideBy;
        if(isNaN(sum)) return false;
        weight.value = sum;
    }

I honestly don’t grasp why it is failing, It would be much appreciated if someone could help me out with this.

Answer 1

var sum = parseInt(document.getElementById('price'+num).value) / divideBy;

You are forcing price into an integer before dividing it. So if price is 3.20, you are actually dividing 3 / 0.32, which is 9.375.

Don’t force it into an integer.