// Converts a character to a value between 0 and 25
function toIndex(char) {
  return (char.charCodeAt(0) + 26 - 97) % 26;
}

// Converts a number from 0 to 25 back into a character
function fromIndex(idx) {
  return String.fromCharCode(((idx + 26) % 26) + 97);
}

function encode(message, password) {
  var encoded = "";
  for (var i = 0; i < message.length; i++) {
    encoded += fromIndex(toIndex(message[i]) + toIndex(password[i % password.length]));
  }
  return encoded;
}

function decode(message, password) {
  var decoded = "";
  for (var i = 0; i < message.length; i++) {
    decoded += fromIndex(toIndex(message[i]) - toIndex(password[i % password.length]));
  }
  return decoded;
}

var message = 'helloiamapandabearhowareyou';
var encoded_message = encode(message, 'teddy');
var decoded_message = decode(encoded_message, 'teddy');
console.log('Original message: ' + message);
console.log('Encoded message: ' + encoded_message);
console.log('Decoded message: ' + decoded_message);