Basic Caesar Cipher and Decipher (send ciphered e-mails and texts with this simple code)

CaesarCipher.js

var cipher = function(str, num) {

    var code = 0,
        cipher = '';
    for (var i = 0; i < str.length; i++) {
        code = str.charCodeAt(i);
        //check lowercase
        if (code >= 97 && code <= 122) {
            code += num;
            //start at beginning of lowcase alphabet
            if (code > 122) {
                code -= 26; 
            }
        }
        //check uppercase
        else if (code >= 65 && code <= 90) {
            code += num;
            //start at beginning of upcase alphabet
            if (code > 90) {
                code -= 26; 
            }
        }
        cipher += String.fromCharCode(code);
    }
    return cipher;
};


var decipher = function(str, num) {

    var code = 0,
        decipher = '';
    for (var i = 0; i < str.length; i++) {
        code = str.charCodeAt(i);
        //check lowercase
        if (code >= 97 && code <= 122) {
            code -= num;
            //start at beginning of lowcase alphabet
            if (code < 97) {
                code += 26; 
            }
        }
        //check uppercase
        else if (code >= 65 && code <= 90) {
            code -= num;
            //start at beginning of upcase alphabet
            if (code < 65) {
                code += 26; 
            }
        }
        decipher += String.fromCharCode(code);
    }
    return decipher;
};

//tests
console.log(cipher('The Fibonacci sequence is named after Fibonacci. His book Liber Abaci introduced the sequence to Western European mathematics, although the sequence had been described earlier in Indian mathematics.', 8))
//Bpm Nqjwvikkq amycmvkm qa viuml inbmz Nqjwvikkq. Pqa jwws Tqjmz Ijikq qvbzwlckml bpm amycmvkm bw Emabmzv Mczwxmiv uibpmuibqka, itbpwcop bpm amycmvkm pil jmmv lmakzqjml miztqmz qv Qvlqiv uibpmuibqka.
console.log(cipher('Hello World!!!', 3)); //Khoor Zruog!!!
console.log(cipher('Biff Tannin', 12)); //Nurr Fmzzuz
console.log(cipher('Carmen San Diego', 8)); //Kizumv Aiv Lqmow

//test decipher
console.log(decipher('Bpm Nqjwvikkq amycmvkm qa viuml inbmz Nqjwvikkq. Pqa jwws Tqjmz Ijikq qvbzwlckml bpm amycmvkm bw Emabmzv Mczwxmiv uibpmuibqka, itbpwcop bpm amycmvkm pil jmmv lmakzqjml miztqmz qv Qvlqiv uibpmuibqka.', 8)); 
//The Fibonacci sequence is named after Fibonacci. His book Liber Abaci introduced the sequence to Western European mathematics, although the sequence had been described earlier in Indian mathematics.