factorial
Calculates the factorial of a number.
Use recursion.
If n is less than or equal to 1, return 1.
Otherwise, return the product of n and the factorial of n - 1.
Throws an exception if n is a negative number.
typescript
const factorial = (n: number): number => (n <= 1 ? 1 : n * factorial(n - 1));
typescript
factorial(6); // 720