Ciphers_Vigenère_Basic.js

/**
 * Check if the Character is letter or not
 * @param {String} str - character to check
 * @return {object} An array with the character or null if isn't a letter
 */
 function isLetter (str) {
    return str.length === 1 && str.match(/[a-zA-Z]/i)
  }
   
  /**
   * Check if is Uppercase or Lowercase
   * @param {String} character - character to check
   * @return {Boolean} result of the checking
   */
  function isUpperCase (character) {
    if (character === character.toUpperCase()) {
      return true
    }
    if (character === character.toLowerCase()) {
      return false
    }
  }
   
  /**
   * Encrypt a Vigenere cipher
   * @param {String} message - string to be encrypted
   * @param {String} key - key for encrypt
   * @return {String} result - encrypted string
   */
  function encrypt (message, key) {
    let result = ''
   
    for (let i = 0, j = 0; i < message.length; i++) {
      const c = message.charAt(i)
      if (isLetter(c)) {
        if (isUpperCase(c)) {
          result += String.fromCharCode((c.charCodeAt(0) + key.toUpperCase().charCodeAt(j) - 2 * 65) % 26 + 65) // A: 65
        } else {
          result += String.fromCharCode((c.charCodeAt(0) + key.toLowerCase().charCodeAt(j) - 2 * 97) % 26 + 97) // a: 97
        }
      } else {
        result += c
      }
      j = ++j % key.length
    }
    return result
  }
   
  /**
   * Decrypt a Vigenere cipher
   * @param {String} message - string to be decrypted
   * @param {String} key - key for decrypt
   * @return {String} result - decrypted string
   */
  function decrypt (message, key) {
    let result = ''
   
    for (let i = 0, j = 0; i < message.length; i++) {
      const c = message.charAt(i)
      if (isLetter(c)) {
        if (isUpperCase(c)) {
          result += String.fromCharCode(90 - (25 - (c.charCodeAt(0) - key.toUpperCase().charCodeAt(j))) % 26)
        } else {
          result += String.fromCharCode(122 - (25 - (c.charCodeAt(0) - key.toLowerCase().charCodeAt(j))) % 26)
        }
      } else {
        result += c
      }
      j = ++j % key.length
    }
    return result
  }
   
  const messageEncrypt = encrypt('Hello World!', 'code')
  console.log(messageEncrypt) // "Jhpnr Yrvng!"
   
  const messageDecrypt = decrypt('Jsopq Zstzg!', 'code')
  console.log(messageDecrypt) // "Hello World!"