capitalize a sentence assuming one space delimiter

capitalizeSentence

// take the str parameter being passed and capitalize the first letter of each word.
// Words will be separated by only one space.
function capitalize(str) {
    var upperLowerDiff = "a".charCodeAt(0) - "A".charCodeAt(0);
    var toUpper = function(c) {
        return String.fromCharCode(c.charCodeAt(0) - upperLowerDiff);
    };
    var isLower = function(c) {
         return /[a-z]/g.test(c);
    };
    return str.split(' ')
                .map(function (word) {
                    var firstLetter = word.charAt(0);
                    return (!isLower(firstLetter))
                        ? word
                        : (word.length == 1)
                            ? toUpper(firstLetter)
                            : toUpper(firstLetter) + word.substring(1);
                })
                .join(' ');
}
String.prototype.titleCase = function() {
    return capitalize(this);
};