Refactor Maths/Factorial for more minimal side effects.

Maths/Factorial.js

@@ -14,36 +14,24 @@
 'use strict'
 
 const calcRange = (num) => {
-  // Generate a range of numbers from 1 to `num`.
-  let i = 1
-  const range = []
-  while (i <= num) {
-    range.push(i)
-    i += 1
-  }
-  return range
+  return [...Array(num).keys()].map(i => i + 1);
 }
 
 const calcFactorial = (num) => {
-  let factorial
-  const range = calcRange(num)
 
-  // Check if the number is negative, positive, null, undefined, or zero
-  if (num < 0) {
-    return 'Sorry, factorial does not exist for negative numbers.'
-  }
-  if (num === null || num === undefined) {
-    return '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 `The factorial of ${num} is ${factorial}`
   }
   if (num === 0) {
     return 'The factorial of 0 is 1.'
   }
-  if (num > 0) {
-    factorial = 1
-    range.forEach(function (i) {
-      factorial = factorial * i
-    })
-    return `The factorial of ${num} is ${factorial}`
+  if (num < 0) {
+    return 'Sorry, factorial does not exist for negative numbers.'
+  }
+  if (!num) {
+    return 'Sorry, factorial does not exist for null or undefined numbers.'
   }
 }
 

Maths/test/Factorial.test.js

@@ -1,9 +1,6 @@
 import { calcFactorial } from '../Factorial'
 
 describe('calcFactorial', () => {
-  it('is a function', () => {
-    expect(typeof calcFactorial).toEqual('function')
-  })
 
   it('should return a statement for value "0"', () => {
     expect(calcFactorial(0)).toBe('The factorial of 0 is 1.')