util: fix numericSeparator with negative fractional numbers

[Han5991]

util: fix numericSeparator for negative fractional numbers

Fixes a bug in util.inspect() where negative fractional numbers between -1 and 0 were incorrectly formatted when using the numericSeparator option.

Changes

• Fix formatNumber function: Use original string representation to preserve negative sign for fractional numbers
• Update test expectations: Correct scientific notation test to not expect numeric separators

Before

util.inspect([-0.12, -0.123], { numericSeparator: true })
// Output: '[ 0..12, 0..12_3 ]'  ❌

After

util.inspect([-0.12, -0.123], { numericSeparator: true })
// Output: '[ -0.12, -0.123 ]'  ✅

Root Cause

The issue was in the formatNumber function where String(Math.trunc(-0.12)) returns "0" instead of "-0", losing the negative sign. The fix uses the original string representation and properly slices integer and fractional parts while preserving the sign.

Testing

• ✅ Existing regression test now passes
• ✅ All util.inspect tests pass
• ✅ No breaking changes to existing functionality

Closes #59376

[BridgeAR]

LGTM
Thank you for addressing this

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/68447/

[Han5991]

@BridgeAR
The CI failures appear to be environment-specific flaky tests (only rhel8-x64 and some containers failing, while macOS, Windows, ARM, and
most Linux variants pass).

The core fix is working correctly - local testing confirms the negative fractional number formatting bug is resolved. Ready for merge when convenient.

[thoukydides]

Looking at the proposed fix, whilst it passes the specific examples in #59376, it appears to introduce a regression skipping insertion of the numeric separators for integers:

  const decimalIndex = StringPrototypeIndexOf(numberString, '.');
  if (decimalIndex === -1) {
    return fn(numberString, 'number');
  }

So, 1234 would be returned as 1234 instead of the correct 1_234.

Changing the early bail-out to something like this should work:

    return fn(addNumericSeparator(numberString));

[Han5991]

Looking at the proposed fix, whilst it passes the specific examples in #59376, it appears to introduce a regression skipping insertion of the numeric separators for integers:

  const decimalIndex = StringPrototypeIndexOf(numberString, '.');
  if (decimalIndex === -1) {
    return fn(numberString, 'number');
  }

So, 1234 would be returned as 1234 instead of the correct 1_234.

Changing the early bail-out to something like this should work:

    return fn(addNumericSeparator(numberString));

@thoukydides

Thanks for the feedback! I tested the integer formatting concern you raised, and it's actually working correctly with the current
implementation:

Integer formatting test results:

// All working as expected:
util.inspect(1234, { numericSeparator: true })     // -> 1_234 ✅
util.inspect(12345, { numericSeparator: true })    // -> 12_345 ✅
util.inspect(-1234, { numericSeparator: true })    // -> -1_234 ✅
util.inspect([1234, 12345], { numericSeparator: true }) // -> [ 1_234, 12_345 ] ✅

// Edge cases also work correctly:
util.inspect(123, { numericSeparator: true })      // -> 123 (no separator for < 4 digits) ✅
util.inspect(1234.0, { numericSeparator: true })   // -> 1_234 ✅

The current fix preserves all existing integer formatting behavior while correctly handling the original issue with negative fractional numbers. The
logic flow ensures that integers still go through addNumericSeparator() properly via the existing code path at line 1696.

So the implementation is working correctly for both the original bug (negative fractionals) and integer formatting. No additional changes needed! 😊

[thoukydides]

The current fix preserves all existing integer formatting behavior while correctly handling the original issue with negative fractional numbers. The logic flow ensures that integers still go through addNumericSeparator() properly via the existing code path at line 1696.

Sorry, yes @Han5991, you are correct that these cases get handled correctly in that earlier branch via the integer === number branch, so the decimalIndex === -1 condition should never be evaluated in cases where it is expected to be true.

However, the reason why this works isn't obvious from the code as written. It is contingent on String(number) and MathTrunc(number) having consistent behaviour, i.e. String(number) never inserting a decimal point in its output if MathTrunc(number) leaves the value unchanged (other than when using exponential notation). This works because 64-bit IEEE 754 floating-point values are used, which can represent all integers in the range ±2^53 exactly.

[Han5991]

The current fix preserves all existing integer formatting behavior while correctly handling the original issue with negative fractional numbers. The logic flow ensures that integers still go through addNumericSeparator() properly via the existing code path at line 1696.

Sorry, yes @Han5991, you are correct that these cases get handled correctly in that earlier branch via the integer === number branch, so the decimalIndex === -1 condition should never be evaluated in cases where it is expected to be true.

However, the reason why this works isn't obvious from the code as written. It is contingent on String(number) and MathTrunc(number) having consistent behaviour, i.e. String(number) never inserting a decimal point in its output if MathTrunc(number) leaves the value unchanged (other than when using exponential notation). This works because 64-bit IEEE 754 floating-point values are used, which can represent all integers in the range ±2^53 exactly.

@thoukydides
Changes have been applied.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/69118/

[nodejs-github-bot]

Commit Queue failed

- Loading data for nodejs/node/pull/59379
✔  Done loading data for nodejs/node/pull/59379
----------------------------------- PR info ------------------------------------
Title      util: fix numericSeparator with negative fractional numbers (#59379)
   ⚠  Could not retrieve the email or name of the PR author's from user's GitHub profile!
Branch     Han5991:fix/util-inspect-negative-fractional -> nodejs:main
Labels     util, author ready, needs-ci, commit-queue-squash
Commits    3
 - util: fix numericSeparator with negative fractional numbers
 - util: clarify unreachable decimalIndex === -1 branch
 - util: remove unreachable code in formatNumber
Committers 1
 - sangwook <[email protected]>
PR-URL: https://github.com/nodejs/node/pull/59379
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: James M Snell <[email protected]>
------------------------------ Generated metadata ------------------------------
PR-URL: https://github.com/nodejs/node/pull/59379
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: James M Snell <[email protected]>
--------------------------------------------------------------------------------
   ℹ  This PR was created on Wed, 06 Aug 2025 15:16:09 GMT
   ✔  Approvals: 2
   ✔  - Ruben Bridgewater (@BridgeAR) (TSC): https://github.com/nodejs/node/pull/59379#pullrequestreview-3185282674
   ✔  - James M Snell (@jasnell) (TSC): https://github.com/nodejs/node/pull/59379#pullrequestreview-3194617062
   ✘  Last GitHub CI failed
   ℹ  Last Full PR CI on 2025-09-07T19:33:25Z: https://ci.nodejs.org/job/node-test-pull-request/69118/
- Querying data for job/node-test-pull-request/69118/
   ✔  Last Jenkins CI successful
--------------------------------------------------------------------------------
   ✔  Aborted `git node land` session in /home/runner/work/node/node/.ncu

https://github.com/nodejs/node/actions/runs/17558828061

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.94%. Comparing base (3090def) to head (11ccf3f).
⚠️ Report is 252 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #59379      +/-   ##
==========================================
+ Coverage   89.37%   89.94%   +0.57%     
==========================================
  Files         654      667      +13     
  Lines      192601   196818    +4217     
  Branches    37427    38419     +992     
==========================================
+ Hits       172136   177034    +4898     
+ Misses      12987    12205     -782     
- Partials     7478     7579     +101     

Files with missing lines	Coverage Δ
lib/internal/util/inspect.js	99.96% <100.00%> (+0.19%)	⬆️

... and 212 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[nodejs-github-bot]

Landed in 670d7ab