VigenereCipher.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
}

export { encrypt, decrypt }

// > encrypt('Hello World!', 'code')
// 'Jsopq Zstzg!'

// > decrypt('Jsopq Zstzg!', 'code')
// 'Hello World!'