jQuery, javascript - How add decimal numbers?

Question

I have created function after button click:

var cena_poczatkowa = parseFloat($("#cena_aktualna").text());
    var cena_dodana = cena_poczatkowa + 1.01;
    $("span#cena_aktualna").text(cena_dodana);

And span in html:

<span id="cena_aktualna">0.00</span>

Everything working fine, after every click number is changed in span: 1.01, 1.02. But after thrid click I see 3.0300000000000002. After fourth click I see again properly 4.04. Why after third click I see this strange number?

Here is my working script so you can see this bug: http://jsfiddle.net/H3pfH/

Possible duplicate (stackoverflow.com/questions/1458633/…). — maerics
– maerics, Commented Aug 23, 2011 at 12:55

Matt · Accepted Answer · 2011-08-23 13:01:16Z

5

Since floating point math is inherently imprecise, try using toFixed() to round it to a suitable number of digits:

var cena_dodana = (cena_poczatkowa + 1.01).toFixed(4);

http://jsfiddle.net/H3pfH/1/

edited Aug 23, 2011 at 13:01

Matt

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answered Aug 23, 2011 at 12:59

Blazemonger

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2 Comments

Cipi Over a year ago

+1 for using toFixed(), but I'd suggest fixing it when there is the need to display the result, and not when calculating the number data, like so: ` $("span#cena_aktualna").text(cena_dodana.toFixed(2));`

Blazemonger Over a year ago

Good point. It all depends on what said number is being used for elsewhere in the code. In this case, I would prefer to round it immediately since it corrects an error and makes the result more correct. If I were taking square roots or some other computation that I would expect to produce a long decimal value, I would round it at the end.

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