Create factorial.js

This program calculates and displays the factorial for a user-input number.

maths/factorial.js

@@ -0,0 +1,52 @@
+/*
+    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
+*/
+
+function calc_range(num) {
+    // Generate a range of numbers from 1 to `num`.
+    "use strict";
+    var i = 1;
+    var range = [];
+    while (i <= num) {
+        range.push(i);
+        i += 1;
+    }
+    return range;
+}
+
+function calc_factorial(num) {
+    "use strict";
+    var factorial;
+    var range = calc_range(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) {
+        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;
+    }
+}
+
+// Run `factorial` Function to find average of a list of numbers.
+var num = prompt("Enter a number: ");
+alert(calc_factorial(num));