Question

Possible Duplicate:
Is JavaScript’s Floating-Point Math Broken?

I have a strange mathematical problem during a multiplication in javascript.

$(parent).find('#id_deals-' + i + '-quantity').val()

result -> 10

$(parent).find('#id_deals-' + i + '-price').val()

result -> 3.99

Both above mulltiplied like this:

$(parent).find('#id_deals-' + i + '-price').val() * $(parent).find('#id_deals-' + i + '-quantity').val()

result –> 39.900000000000006

Why is this happening? and what can I do to limit the decimal places to 2 digits only?

Is it maybe because 10 has to be 10.0 ? But how do I convert my value to this format automatically before the actual multiplication?

Update:
According to syazdani’s answer, I have tried to implement bigdecimal as suggested:

It is not well documented, but I got it working like this:

function run(opts) {
    var bd = {"BigDecimal":BigDecimal, "BigInteger":BigInteger, "RoundingMode":RoundingMode};
    var result;
    var ops = {'*': "multiply", '/': "divide", '+': "add", '-': "subtract"};
    var a = new bd.BigDecimal("" + opts.a);
    var b = new bd.BigDecimal("" + opts.b);
    var op = ops[opts.op];
    if (op == "divide") {
        return a.divide(b, 300, bd.RoundingMode.HALF_UP());
    } else {
        return a[op].call(a, b);
    }
}


function multiply(a, b){
    return run({"a":a,"b":b,"op":"*"}); 
} 

Answer 1

If you are working with currency (as it seems that you are given the “price” id), you may be better served by using a so called Big Number library (such as this one: https://github.com/iriscouch/bigdecimal.js) for your math to control the math (round up vs round down, etc.). It takes a bit more work to get everything right, but it is worthwhile to avoid the Office Space math scenario.