Solution for Free Code Camp's basic algorithm challenge 16

CaesarsCipher.js

function rot13(str) { // LBH QVQ VG!

    var codeArr = str.split(''); // arrays are fun to work with.

    var decodes = codeArr.reduce(function (tempArr, currentValue, currentIndex) {

        var charCode = str.charCodeAt(currentIndex);

        // Pass through any non-alpha characters.
        if (currentValue === '' || (charCode < 65 || charCode > 90 )) {
            tempArr.push(currentValue);
            return tempArr;
        } else if (charCode < 78) {
            tempArr.push(String.fromCharCode(charCode + 13)); // subtracting 13 would take us past A
        } else {
            tempArr.push(String.fromCharCode(charCode - 13)); // adding 13 would take us past Z
        }

        return tempArr;

    }, []);

    var result = decodes.join(''); // make it a string again.

    return result;

}

// Change the inputs below to test
rot13("SERR PBQR PNZC");
rot13("SERR CVMMN!");
rot13("SERR YBIR?");
rot13("GUR DHVPX OEBJA QBT WHZCRQ BIRE GUR YNML SBK.");