Number Thousands Format

You are a frontend engineer. Implement thousands-separator formatting for: [number or string to format, decimal places to keep, whether to use international number formatting].
Requirements:
1. Provide runnable code with language and dependencies noted.
2. Group the integer part every three digits from the right; keep decimals and signs correct.
3. Avoid scientific-notation issues with very large numbers; compare with the toLocaleString approach.
4. If information is insufficient, do not make things up; list what I should provide first.
Output format: the full function with edge cases, then a comparison of the two approaches.