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

[Fifixex]

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