This PR improves the existing factorial(n) function by adding input validation for negative numbers. Previously, calling factorial(-1) or any negative input would cause infinite recursion, eventually leading to a stack overflow. With this update, a RangeError is thrown when a negative number is passed.

Why this change is important:

• Correctness: Mathematically, factorial is not defined for negative numbers.
• Robustness: Prevents infinite recursion and potential stack overflow on invalid input.
• Interview relevance: Input validation is a common area of oversight in candidate solutions. This change aligns the function with real-world expectations.
• Clarity and semantics: Using RangeError instead of a generic Error makes it immediately clear that the issue is with the range of acceptable values, not a syntax issue or unexpected type. According to MDN, RangeError is appropriate when a value is not within the set or range of allowed values — which fits this case perfectly.

factorial(-5); // throws RangeError: Factorial is not defined for negative numbers
factorial(5); // returns 120

The updated function stays concise and educational while being safer and more professional.