Factorial Recursion Challenge

Given a non-negative integer n, return its factorial. The factorial of n is the product of all positive integers less than or equal to n. If n is negative, return null.

Function Signature

factorial(n)

Parameters

• n — an integer

Output

Return a number representing the factorial of n, or null if n is negative.

Constraints

• The function must use recursion.
• The function should accept exactly one argument.

Examples

factorial(0)  → 1
factorial(1)  → 1
factorial(4)  → 24
factorial(5)  → 120
factorial(-5) → null