Javascript: formatting a rounded number to N decimals

0 votes
asked Feb 8, 2010 by hoju

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?

9 Answers

0 votes
answered Feb 8, 2010 by guffa

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.)

0 votes
answered Feb 8, 2010 by paul

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?

0 votes
answered Feb 8, 2010 by christoph

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;
}
0 votes
answered Feb 25, 2010 by elias-zamaria

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

function toFixed(value, precision) {
    var precision = precision || 0,
        power = Math.pow(10, precision),
        absValue = Math.abs(Math.round(value * power)),
        result = (value < 0 ? '-' : '') + String(Math.floor(absValue / power));

    if (precision > 0) {
        var fraction = String(absValue % power),
            padding = new Array(Math.max(precision - fraction.length, 0) + 1).join('0');
        result += '.' + padding + fraction;
    }
    return result;
}

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

0 votes
answered Feb 18, 2011 by david

I think that there is a more simple approach to all given here, and is the method Number.toFixed() already implemented in JavaScript.

simply write:

var myNumber = 2;

myNumber.toFixed(2); //returns "2.00"
myNumber.toFixed(1); //returns "2.0"

etc...

0 votes
answered Feb 16, 2012 by kirill

There's always a better way for doing things.

var number = 51.93999999999761;

I would like to get four digits precision: 51.94

just do:

number.toPrecision(4);

the result will be: 51.94

0 votes
answered Feb 16, 2014 by juan-carrey

If you do not really care about rounding, just added a toFixed(x) and then removing trailing 0es and the dot if necessary. It is not a fast solution.

function format(value, decimals) {
    if (value) {
        value = value.toFixed(decimals);            
    } else {
        value = "0";
    }
    if (value.indexOf(".") < 0) { value += "."; }
    var dotIdx = value.indexOf(".");
    while (value.length - dotIdx <= decimals) { value += "0"; } // add 0's

    return value;
}
0 votes
answered Feb 16, 2016 by johnnybizzle

PHP-Like rounding Method

The code below can be used to add your own version of Math.round to your own namespace which takes a precision parameter. Unlike Decimal rounding in the example above, this performs no conversion to and from strings, and the precision parameter works same way as PHP and Excel whereby a positive 1 would round to 1 decimal place and -1 would round to the tens.

var myNamespace = {};
myNamespace.round = function(number, precision) {
    var factor = Math.pow(10, precision);
    var tempNumber = number * factor;
    var roundedTempNumber = Math.round(tempNumber);
    return roundedTempNumber / factor;
};

myNamespace.round(1234.5678, 1); // 1234.6
myNamespace.round(1234.5678, -1); // 1230

from Mozilla Developer reference for Math.round()

0 votes
answered Sep 15, 2017 by krishna-teja

I think below function can help

function roundOff(value,round) {
   return (parseInt(value * (10 ** (round + 1))) - parseInt(value * (10 ** round)) * 10) > 4 ? (((parseFloat(parseInt((value + parseFloat(1 / (10 ** round))) * (10 ** round))))) / (10 ** round)) : (parseFloat(parseInt(value * (10 ** round))) / ( 10 ** round));
}

usage : roundOff(600.23458,2); will return 600.23

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