const rows = ['qwertyuiopå', 'asdfghjklöä', 'zxcvbnm'];
const charLists = [
  ...rows,
  ...rows.map(row => row.toUpperCase()),
  '1234567890'
];

const transform = transformer => text =>
  text.split('').map(char => {
    const chars = charLists.find(list => list.includes(char));
    return chars
      ? transformer(chars.indexOf(char), chars)
      : char;
  }).join('');

const shiftRight = transform((index, chars) => {
  const i = index + 1;
  return i < chars.length
    ? chars[i]
    : chars[0];
});

const shiftLeft = transform((index, chars) => {
  const i = index - 1;
  return i < 0
    ? chars[chars.length - 1]
    : chars[i];
});

console.log(shiftRight('Moi! testiä 123')); // => "Zpo! yrdyoa 234"
console.log(shiftLeft('Zpo! yrdyoa 234')); // => "Moi! testiä 123"