Rounding decimal numbers in Javascript

Question

Every javascript programmer knows that javascript Math.round() does not produce correct rounding for decimal numbers in many cases, due to faulty floating point arithmetics.

Most found solutions tell to use simple prototype extension:

Number.prototype.round = function (decimalPlaces) {
    decimalPlaces = decimalPlaces || 0;
    var power = Math.pow(10, decimalPlaces)

    if (this > 0) {
        return Math.round(this * power) / power;
    } else {
        return -(Math.round(Math.abs(this) * power) / power);
    }
};

But this one fails on (1.005).round(2), which returns 1 instead of expected 1.01.

So how to correctly round decimal numbers in javascript? See the solution bellow (with unit tests).

Personally I find it best to just work with integers, and only divide by 100 or whatever for display purposes ;) — Niet the Dark Absol, Jan 12 '16 at 09:17

score 0 · Answer 1 · edited Jan 12 '16 at 09:16

Correct Number prototype extension for rounding decimal numbers:

Number.prototype.round = function (decimalPlaces) {
    decimalPlaces = decimalPlaces || 0;

    var sign = (this < 0) ? -1 : 1,
        exp = -decimalPlaces,
        value;

    // Shift
    value = Math.abs(this).toString().split('e');
    value = Math.round(+(value[0] + 'e' + (value[1] ? (+value[1] - exp) : -exp)));

    // Shift back
    value = value.toString().split('e');

    return (+(value[0] + 'e' + (value[1] ? (+value[1] + exp) : exp))) * sign;
};

This extension is a tweek of decimalAdjust(), mentioned at the Mozilla Developer Network Documentation for Math.round() :

QUnit tests for this solution:

test("Number.prototype.round", function () {
    var value;

    value = 0;
    equal(value.round(0), 0, "Value: 0, decimals: 0, expected result: 0");

    value = 33.12;
    equal(value.round(1), 33.1, "Value: 33.12, decimals: 1, expected result: 33.1");

    value = 33.15;
    equal(value.round(1), 33.2, "Value: 33.15, decimals: 1, expected result: 33.2");

    value = 33.123;
    equal(value.round(2), 33.12, "Value: 33.123, decimals: 2, expected result: 33.12");

    value = 1000033.125;
    equal(value.round(2), 1000033.13, "Value: 1000033.125, decimals: 2, expected result: 1000033.13");

    value = 17.6949;
    equal(value.round(2), 17.69, "Value: 17.6949, decimals: 2, expected result: 17.69");

    value = 33.1234;
    equal(value.round(3), 33.123, "Value: 33.1234, decimals: 3, expected result: 33.123");

    value = 33.12359;
    equal(value.round(3), 33.124, "Value: 33.12359, decimals: 3, expected result: 33.124");

    value = 33.12343;
    equal(value.round(4), 33.1234, "Value: 33.12343, decimals: 4, expected result: 33.1234");

    value = 33.12346;
    equal(value.round(4), 33.1235, "Value: 33.12346, decimals: 4, expected result: 33.1235");

    value = 33.123456789;
    equal(value.round(5), 33.12346, "Value: 33.123456789, decimals: 5, expected result: 33.12346");

    value = 33.123456789;
    equal(value.round(8), 33.12345679, "Value: 33.123456789, decimals: 8, expected result: 33.12345679");

    value = -123456.123456789;
    equal(value.round(8), -123456.12345679, "Value: -123456.123456789, decimals: 8, expected result: -123456.12345679");

    value = -110.385;
    equal(value.round(2), -110.39, "Value: -110.385, decimals: 2, expected result: -110.39");
});

You shouldn't extend native objects in JavaScript -> http://stackoverflow.com/questions/14034180/why-is-extending-native-objects-a-bad-practice — John Slegers, Jan 12 '16 at 09:20
Well, this prototype extension can be easily rewritten to standard function. — petriq, Jan 12 '16 at 13:33

Rounding decimal numbers in Javascript

1 Answers1