spokodev · 2026-06-30T11:46:38Z

Problem

The integer part of a large integral value changes depending only on the requested mantissa length:

numbro(9.999999e20).format({mantissa: 0, trimMantissa: false}) // '999999900000000000000'        correct
numbro(9.999999e20).format({mantissa: 2, trimMantissa: false}) // '999999900000000000000.00'     correct
numbro(9.999999e20).format({mantissa: 3, trimMantissa: false}) // '999999899999999900000.000'    corrupted
numbro(9.999999e20).format({mantissa: 4, trimMantissa: false}) // '999999899999999900000.0000'   corrupted

An integral value's integer part must be invariant to the requested mantissa length, so the 999999899999999900000 results are wrong.

Root cause

toFixed in src/formatting.js scales, rounds and divides in native float:

const n = new BigNumber(roundingFunction(+`${value}e+${precision}`) / (Math.pow(10, precision)));

For an integral value above 2^53, scaling by 10^precision and dividing back in native float loses precision. The result flips between correct and corrupted purely as a function of the requested decimal count.

Fix

bignumber.js is already imported in this module. The scale and divide now run through BigNumber, so large values keep their digits. The user-supplied roundingFunction is still applied, so rounding behavior is unchanged.

cons
9FFB
t power = new BigNumber(10).pow(precision);
const rounded = roundingFunction(new BigNumber(value).times(power).toNumber());
const n = new BigNumber(rounded).div(power);
return n.toFixed(precision);

Test

Added a regression test in the toFixed block asserting the integer part of 9.999999e20 stays 999999900000000000000 for mantissa 0..6. It fails on the current code (mantissa 3, 4, 6) and passes with the fix. Full unit suite stays green (463 specs, 0 failures); integration suite green (2 specs, 0 failures).

`toFixed` scaled, rounded and divided the value in native float (`roundingFunction(+`${value}e+${precision}`) / Math.pow(10, precision)`). For integral values above 2^53 this loses precision, so the integer part of the output changed depending only on the requested mantissa length. Example with 9.999999e20: format({mantissa: 0}) -> 999999900000000000000 (correct) format({mantissa: 2}) -> 999999900000000000000.00 (correct) format({mantissa: 3}) -> 999999899999999900000.000 (corrupted) format({mantissa: 4}) -> 999999899999999900000.0000 (corrupted) An integral value's integer part must be invariant to the requested mantissa length. bignumber.js is already imported in this module, so the scale and divide now run through BigNumber and large values keep their digits. Adds a regression test asserting the integer part stays constant for mantissa 0..6.

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Uh oh!

fix: avoid native float overflow when scaling large values in toFixed - #777