util.inspect: incorrect numericSeparator formatting for numbers in scientific notation (e.g. 1e-7)

Version

v24.14.0

Platform

Linux DESKTOP-59R7A1D 6.6.87.2-microsoft-standard-WSL2 #1 SMP PREEMPT_DYNAMIC Thu Jun  5 18:30:46 UTC 2025 x86_64 GNU/Linux

Subsystem

util.inspect

What steps will reproduce the bug?

const util = require('util');

console.log(util.inspect(1e-7, { numericSeparator: true }));

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

Always reproducible when:

• numericSeparator is set to true
• The number is formatted using scientific notation (e.g. 1e-7, 1e+21)

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

The output should remain a valid numeric string representation.

For example:

'1e-7'

Scientific notation should either:

• remain unchanged, or
• be properly expanded before applying numeric separators

But it should never produce malformed output.

What do you see instead?

Malformed and corrupted output:

'1e-.1e-_7'

This happens because the formatting logic assumes that any non-integer number contains a decimal point (.), and attempts to split the string accordingly. However, scientific notation strings like '1e-7' do not contain a decimal point, leading to incorrect string slicing and separator insertion.

Additional information

No response

Start with the util.inspect entry point and reproduce the provided numericSeparator example using Node.js. Trace the formatting path for scientific notation and verify that the result remains a valid numeric string; add regression coverage for both 1e-7 and 1e+21 if the surrounding tests provide a suitable location.