Factorial.js

/*
 author: PatOnTheBack
 license: GPL-3.0 or later

 Modified from:
 https://github.com/TheAlgorithms/Python/blob/master/maths/factorial_python.py

 This script will find the factorial of a number provided by the user.

 More about factorials:
 https://en.wikipedia.org/wiki/factorial
*/

'use strict'

const calcRange = (num) => {
 return [...Array(num).keys()].map((i) => i + 1)
}

const calcFactorial = (num) => {
 if (num === 0) {
 return 1
 }
 if (num < 0) {
 throw Error('Sorry, factorial does not exist for negative numbers.')
 }
 if (!num) {
 throw Error(
 'Sorry, factorial does not exist for null or undefined numbers.'
 )
 }
 if (num > 0) {
 const range = calcRange(num)
 const factorial = range.reduce((a, c) => a * c, 1)
 return factorial
 }
}

export { calcFactorial }