Gaussian/banker's rounding in JavaScript

I have been using Math.Round(myNumber, MidpointRounding.ToEven) in C# to do my server-side rounding, however, the user needs to know 'live' what the result of the server-side operation will be which means (avoiding an Ajax request) creating a JavaScript method to replicate the MidpointRounding.ToEven method used by C#.

MidpointRounding.ToEven is Gaussian/banker's rounding, a very common rounding method for accounting systems described here.

Does anyone have any experience with this? I have found examples online, but they do not round to a given number of decimal places...

function evenRound(num, decimalPlaces) {
    var d = decimalPlaces || 0;
    var m = Math.pow(10, d);
    var n = +(d ? num * m : num).toFixed(8); // Avoid rounding errors
    var i = Math.floor(n), f = n - i;
    var e = 1e-8; // Allow for rounding errors in f
    var r = (f > 0.5 - e && f < 0.5 + e) ?
                ((i % 2 == 0) ? i : i + 1) : Math.round(n);
    return d ? r / m : r;
}

console.log( evenRound(1.5) ); // 2
console.log( evenRound(2.5) ); // 2
console.log( evenRound(1.535, 2) ); // 1.54
console.log( evenRound(1.525, 2) ); // 1.52

Live demo: http://jsfiddle.net/NbvBp/

For what looks like a more rigorous treatment of this (I've never used it), you could try this BigNumber implementation.

This is the unusual stackoverflow where the bottom answers are better than the accepted. Just cleaned up @xims solution and made a bit more legible:

function bankersRound(n, d=2) {
    var x = n * Math.pow(10, d);
    var r = Math.round(x);
    var br = Math.abs(x) % 1 === 0.5 ? (r % 2 === 0 ? r : r-1) : r;
    return br / Math.pow(10, d);
}

That's a great solution from @soegaard. Here is a small change that makes it work for decimal points:

bankers_round(n:number, d:number=0) {
	var x = n * Math.pow(10, d);
	var r = Math.round(x);
	var br = (((((x>0)?x:(-x))%1)===0.5)?(((0===(r%2)))?r:(r-1)):r);
	return br / Math.pow(10, d);
}

And while at it - here are some tests:

console.log(" 1.5 -> 2 : ", bankers_round(1.5) );
console.log(" 2.5 -> 2 : ", bankers_round(2.5) );
console.log(" 1.535 -> 1.54 : ", bankers_round(1.535, 2) );
console.log(" 1.525 -> 1.52 : ", bankers_round(1.525, 2) );

console.log(" 0.5 -> 0 : ", bankers_round(0.5) );
console.log(" 1.5 -> 2 : ", bankers_round(1.5) );
console.log(" 0.4 -> 0 : ", bankers_round(0.4) );
console.log(" 0.6 -> 1 : ", bankers_round(0.6) );
console.log(" 1.4 -> 1 : ", bankers_round(1.4) );
console.log(" 1.6 -> 2 : ", bankers_round(1.6) );

console.log(" 23.5 -> 24 : ", bankers_round(23.5) );
console.log(" 24.5 -> 24 : ", bankers_round(24.5) );
console.log(" -23.5 -> -24 : ", bankers_round(-23.5) );
console.log(" -24.5 -> -24 : ", bankers_round(-24.5) );

The accepted answer does round to a given number of places. In the process it calls toFixed which converts the number to a string. Since this is expensive, I offer the solution below. It rounds a number ending in 0.5 to the nearest even number. It does not handle rounding to an arbitrary number of places.

function even_p(n){
  return (0===(n%2));
};

function bankers_round(x){
    var r = Math.round(x);
    return (((((x>0)?x:(-x))%1)===0.5)?((even_p(r))?r:(r-1)):r);
};

const isEven = (value: number) => value % 2 === 0;
const isHalf = (value: number) => {
    const epsilon = 1e-8;
    const remainder = Math.abs(value) % 1;

    return remainder > .5 - epsilon && remainder < .5 + epsilon;
};

const roundHalfToEvenShifted = (value: number, factor: number) => {
    const shifted = value * factor;
    const rounded = Math.round(shifted);
    const modifier = value < 0 ? -1 : 1;

    return !isEven(rounded) && isHalf(shifted) ? rounded - modifier : rounded;
};

const roundHalfToEven = (digits: number, unshift: boolean) => {
    const factor = 10 ** digits;

    return unshift
        ? (value: number) => roundHalfToEvenShifted(value, factor) / factor
        : (value: number) => roundHalfToEvenShifted(value, factor);
};

const roundDollarsToCents = roundHalfToEven(2, false);
const roundCurrency = roundHalfToEven(2, true);

• If you do not like the overhead of calling toFixed()
• Want to be able to supply an arbitrary scale
• Don't want to introduce floating-point errors
• Want to have readable, reusable code

roundHalfToEven is a function that generates a fixed scale rounding function. I do my currency operations on cents, rather than dollars, to avoid introducing FPEs. The unshift param exists to avoid the overhead of unshifting and shifting again for those operations.

Stricly speaking, all of these implementations should handle the case of a negative number of digits to round to.

It is an edge case, but still it would be wise to disallow it (or be very clear about what that means, for example -2 is rounding to the nearest amount of hundreds).

I was not happy with the other answers. They have either too verbose or complicated code or fail to round properly for negative numbers. For negative numbers we have to cleverly fix a weird behavior of JavaScript:

> JavaScript's Math.round has the unusual property that it rounds halfway cases towards positive infinity, regardless of whether they're positive or negative. So for example 2.5 will round to 3.0, but -2.5 will round to -2.0. Source

This is wrong, so we have to round down on negatives .5 before applying the bankers rounding, accordantly.

Also, just as Math.round, I want to round to the next integer and enforce a precision of 0. I just want Math.round with the correct and fixed "round halves to even" method in positive and negative. It needs to round the same like in other programming languages such as PHP (PHP_ROUND_HALF_EVEN) or C# (MidpointRounding.ToEven).

/**
 * Returns a supplied numeric expression rounded to the nearest integer while rounding halves to even.
 */
function roundMidpointToEven(x) {
  const n = x >= 0 ? 1 : -1 // n describes the adjustment on an odd rounding from midpoint
  const r = n * Math.round(n * x) // multiplying n will fix negative rounding
  return Math.abs(x) % 1 === 0.5 && r % 2 !== 0 ? r - n : r // we adjust by n if we deal with a half on an odd rounded number
}

// testing by rounding cents:
for(let i = -10; i <= 10; i++) {
  const val = i + .5
  console.log(val + " => " + roundMidpointToEven(val))
}

Math.round as well as our custom roundMidpointToEven function won't care for precision, because it's always better to calculate with cents to avoid float-point issues on any calculations anyways.

However, if you don't deal with cents you can simply multiply and divide the appropriate factor for the number of decimal placeholders in the same way you would do it for Math.round:

const usd = 9.225;
const fact = Math.pow(10, 2) // A precision of 2, so 100 is the factor
console.log(roundMidpointToEven(usd * fact) / fact) // outputs 9.22 instead of 9.23

To fully validate the custom roundMidpointToEven function, here is the same output using PHP with its official PHP_ROUND_HALF_EVEN as well as C# using MidpointRounding.ToEven:

for($i = -10; $i <= 10; $i++) {
    $val = $i + .5;
    echo $val . ' => ' . round($val, 0, PHP_ROUND_HALF_EVEN) . "<br />";
}

for(int i = -10; i <= 10; i++) 
{
	double val = i + .5;
	Console.WriteLine(val + " => " + Math.Round(val, MidpointRounding.ToEven));
}

Both snippets return the same like the test call of our custom roundMidpointToEven:

-9.5 => -10
-8.5 => -8
-7.5 => -8
-6.5 => -6
-5.5 => -6
-4.5 => -4
-3.5 => -4
-2.5 => -2
-1.5 => -2
-0.5 => 0
0.5 => 0
1.5 => 2
2.5 => 2
3.5 => 4
4.5 => 4
5.5 => 6
6.5 => 6
7.5 => 8
8.5 => 8
9.5 => 10
10.5 => 10

Success!

For folks who want to be able to read the code a little better, here's an alternative implementation that seems to work.

function bankersRound(n, decimalPlaces) {
  // Create our multiplier for floating point precision issues.
  const multiplier = Math.pow(10, decimalPlaces);
  // Multiple by decimal places to avoid rounding issues w/ floats
  const num = n * multiplier;
  // Use standard rounding
  const rounded = Math.round(num);
  // Only odd numbers should be rounded
  const shouldUseBankersRound = rounded % 2 !== 0;
  // Subtract one to ensure the rounded number is even
  const bankersRound = shouldUseBankersRound ? rounded - 1 : rounded;
  // Return to original precision
  return bankersRound / multiplier;
}

console.log(
  bankersRound(1.5255, 2),
  bankersRound(1.53543, 2),
  bankersRound(1.54543, 2),
  bankersRound(1.54543, 3),
  bankersRound(1.53529, 4),
  bankersRound(1.53529, 2),
  bankersRound(4.5, 0),
  bankersRound(5.5, 0),
  bankersRound(0.045, 2),
  bankersRound(0.055, 2)
);