util.inspect incorrectly formats negative fractional numbers with numericSeparator: true

Version

v24.5.0 (also reproduced on v20.19.2)

Platform

Linux nexus 6.14.0-27-generic #27~24.04.1-Ubuntu SMP PREEMPT_DYNAMIC Tue Jul 22 17:38:49 UTC 2 x86_64 x86_64 x86_64 GNU/Linux

Subsystem

util

What steps will reproduce the bug?

import { inspect } from 'util';

const values = [0.1234, -0.12, -0.123, -0.1234, -1.234];
const text = inspect(values, { numericSeparator: true });
console.log(text);

How often does it reproduce? Is there a required condition?

Always reproducible for negative numbers strictly between -1 and 0, when numericSeparator: true is specified.

What is the expected behavior? Why is that the expected behavior?

The output should be syntactically valid JavaScript numeric literals representing the original values, with numeric separators inserted in accordance with the numericSeparator option. For the sample above, the expected output is::

[ 0.123_4, -0.12, -0.123, -0.123_4, -1.234 ]

What do you see instead?

The actual output of the sample script is:

[ 0.123_4, 0..12, 0..12_3, 0..12_34, -1.234 ]

The three incorrect values:

Are missing their leading minus sign
Contain a spurious double decimal point (0..)
Place the numeric separator incorrectly

Additional information

No response

I believe the bug is caused by the following formatNumber code in lib/internal/util/inspect.js:

  const integer = MathTrunc(number);
  const string = String(integer);
  ...
  return fn(`${
    addNumericSeparator(string)
  }.${
    addNumericSeparatorEnd(
      StringPrototypeSlice(String(number), string.length + 1),
    )
  }`, 'number');
}

For values where -1 < number < 0, this results in:

integer === -0 (correct)
string === '0' (loses the leading minus sign due to coercion)
addNumericSeparator(string) omits the minus sign from the formatted number
StringPrototypeSlice(String(number), string.length + 1) incorrectly retains the original decimal point, resulting in a duplicated dot (..) in the output

One potential fix would be to use logic similar to the non-numericSeparator case earlier in the function:

  const string = ObjectIs(number, -0) ? '-0' : String(integer);

However, a cleaner and more robust approach might be to either:

Remove the sign before applying numeric separators, and prepend the minus sign to the final output if number < 0 || ObjectIs(number, -0); or
Perform the string conversion once, then locate the decimal point (if any) and split the string appropriately for formatting.

I'll give it a try?

@Han5991

I would either go with the quick fix suggested here or by performing the string conversion once.

@Han5991 go ahead. If you open a PR, please reference this issue with Closes #59376.

util: fix numericSeparator with negative fractional numbers

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util.inspect incorrectly formats negative fractional numbers with numericSeparator: true #59376

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What steps will reproduce the bug?

How often does it reproduce? Is there a required condition?

What is the expected behavior? Why is that the expected behavior?

What do you see instead?

Additional information

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util.inspect incorrectly formats negative fractional numbers with numericSeparator: true #59376

Description

Version

Platform

Subsystem

What steps will reproduce the bug?

How often does it reproduce? Is there a required condition?

What is the expected behavior? Why is that the expected behavior?

What do you see instead?

Additional information

Activity

thoukydides commented on Aug 6, 2025

Han5991 commented on Aug 6, 2025

BridgeAR commented on Aug 6, 2025

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