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Factorial Calculator

A simple script to calculate the factorial of a number entered by the user.

  1. // ==UserScript==
  2. // @name         Factorial Calculator
  3. // @namespace    http://example.com/factorialcalculator
  4. // @version      1.0
  5. // @description  A simple script to calculate the factorial of a number entered by the user.
  6. // @author       Your Name
  7. // @match        *://*/*
  8. // @license      MIT
  9. // @grant        none
  10. // @locale       en
  11. // ==/UserScript==
  12.  
  13. (function() {
  14.     'use strict';
  15.  
  16.     /**
  17.      * This function calculates the factorial of a given number 'n'.
  18.      * The factorial of a number is the product of all positive integers less than or equal to 'n'.
  19.      * For example:
  20.      *     - factorial(5) = 5 * 4 * 3 * 2 * 1 = 120
  21.      *     - factorial(0) = 1 (by definition)
  22.      *
  23.      * @param {number} n - The number to calculate the factorial of. Must be a non-negative integer.
  24.      * @returns {number} The factorial of the input number 'n'.
  25.      * @throws {Error} If the input 'n' is negative, as factorial is only defined for non-negative integers.
  26.      */
  27.     function factorial(n) {
  28.         if (n < 0) {
  29.             throw new Error('Input must be a non-negative integer.');
  30.         }
  31.         
  32.         let result = 1;
  33.         for (let i = 1; i <= n; i++) {
  34.             result *= i;
  35.         }
  36.         return result;
  37.     }
  38.  
  39.     // Example usage:
  40.     // Prompt the user to enter a number
  41.     let num = parseInt(prompt('Enter a non-negative integer:', ''), 10);
  42.     
  43.     // Ensure valid input and display result
  44.     if (!isNaN(num) && num >= 0) {
  45.         alert('The factorial of ' + num + ' is ' + factorial(num));
  46.     } else {
  47.         alert('Please enter a valid non-negative integer.');
  48.     }
  49. })();