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.
• Throw a TypeError if n is a negative number.

const factorial = n =>
  n < 0
    ? (() => {
        throw new TypeError('Negative numbers are not allowed!');
      })()
    : n <= 1
    ? 1
    : n * factorial(n - 1);

factorial(6); // 720