//jshint esnext:true

​

console.clear();

​

const code = char => char.charCodeAt(0) - 97;

const operate = (data, keyword, mod) => String.fromCharCode.apply(null, data

    .split('')

    .map((char, idx) => code(char) + (mod * code(keyword[idx % keyword.length])))

    .map(offset => (offset + 26) % 26 + 97));

const encode = (data, keyword) => operate(data, keyword, 1);

const decode = (data, keyword) => operate(data, keyword, -1);

​

console.log(encode('thepackagehasbeendelivered', 'snitch'));

console.log(encode('theredfoxtrotsquietlyatmidnight', 'bond'));

console.log(encode('murderontheorientexpress', 'train'));

console.log(encode('themolessnuckintothegardenlastnight', 'garden'));

console.log(decode('klatrgafedvtssdwywcyty', 'cloak'));

console.log(decode('pjphmfamhrcaifxifvvfmzwqtmyswst', 'python'));

console.log(decode('rcfpsgfspiecbcc', 'moore'));