round number in JavaScript to N decimal places

Question

in JavaScript, the typical way to round a number to N decimal places is something like:

function round_number(num, dec) {
    return Math.round(num * Math.pow(10, dec)) / Math.pow(10, dec);
}

However this approach will round to a maximum of N decimal places while I want to always round to N decimal places. For example "2.0" would be rounded to "2".

Any ideas?

Answer 1

That's not a rounding ploblem, that is a display problem. A number doesn't contain information about significant digits; the value 2 is the same as 2.0000000000000. It's when you turn the rounded value into a string that you have make it display a certain number of digits.

You could just add zeroes after the number, something like:

var s = number.toString();
if (s.indexOf('.') == -1) s += '.';
while (s.length < s.indexOf('.') + 4) s += '0';

(Note that this assumes that the regional settings of the client uses period as decimal separator, the code needs some more work to function for other settings.)

Answer 2

Here's a link to a Javascript sprintf,

http://www.webtoolkit.info/javascript-sprintf.html

A call to sprintf() is one rounding methodology in perl, but javascript doesn't have that function natively.

http://perldoc.perl.org/functions/sprintf.html

Does that help?

Answer 3

Hopefully working code (didn't do much testing):

function toFixed(value, precision) {
    var precision = precision || 0,
        neg = value < 0,
        power = Math.pow(10, precision),
        value = Math.round(value * power),
        integral = String((neg ? Math.ceil : Math.floor)(value / power)),
        fraction = String((neg ? -value : value) % power),
        padding = new Array(Math.max(precision - fraction.length, 0) + 1).join('0');

    return precision ? integral + '.' +  padding + fraction : integral;
}

Answer 4

I found a way. This is Christoph's code with a fix:

function toFixed(value, precision) {
var precision = precision || 0,
    neg = value < 0,
    power = Math.pow(10, precision),
    value = Math.round(value * power),
    integral = String((neg ? Math.ceil : Math.floor)(value / power)),
    fraction = String((neg ? -value : value) % power),
    padding = new Array(Math.max(precision - fraction.length, 0) + 1).join('0');

return precision ? integral + '.' +  padding + fraction : integral;

}

Read the details of repeating a character using an array constructor here if you are curious as to why I added the "+ 1".