Letter Changes

Letter Changes

// take the str parameter being passed and modify it using the 
// following algorithm. 
// Replace every letter in the string with the letter following it in the alphabet 
// (ie.c becomes d, z becomes a).Then capitalize every vowel in this new string (a, e, i, o, u)
// and finally return this modified string. 

function letterChanges(str) {
    var lowerStart = "a".charCodeAt(0);
    var upperStart = "A".charCodeAt(0);
    var letterCount = ("z".charCodeAt(0) - lowerStart) + 1; // 26;
    var lowerUpperDiff = "a".charCodeAt(0) - upperStart; // 32
    var vowels = "aioue";

    var strArray = str.split('');
    var valueMap = strArray.reduce(function (p, c) {
                                p[c] = c.charCodeAt(0);
                                return p;
                            }, {});

    return strArray.map(function nextChar(c) {
                        var isLetter = /\w/g.test(c);
                        if (!isLetter) return c;
                        var isLower = /[a-z]/.test(c);
                        var start = (isLower) ? lowerStart : upperStart;
                        return String.fromCharCode(((valueMap[c] + 1 - start) % letterCount) + start);
                    })
                    .map(function lowerVowelToUpper(c) {
                        var isLetter = /\w/g.test(c);
                        var isVowel = vowels.indexOf(c) >= 0;
                        return (isLetter && isVowel)
                            ? String.fromCharCode(c.charCodeAt(0) - lowerUpperDiff)
                            : c;
                    })
                    .join('');
}