cipher.js

var alphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';

function cipher(input, shift) {
    var result = '';

    for (var i = 0; i < input.length; i++) {
        var position = alphabet.indexOf(input[i].toUpperCase()) + Math.floor(shift);
        if (position > 25) {
            position -= 26;
        }
        result = result.concat(alphabet[position]);
    }

    return result;
}