reverseString.js

function reverseString(str) {
    var chars = str.split(""),
        letterNum = 1,
        start = "A".charCodeAt(0),
        end = "Z".charCodeAt(0),
        list = [];

    function opposite(char) {
        var oppCharCode,
            re = /[A-Za-z]/g;
        if (!re.test(char)) {
            return char;
        }

        oppCharCode = list.filter(function(obj) {
            return obj.letter === char.toUpperCase();
        })[0].oppositeCharCode;
        if (char.charCodeAt(0) > end) {
            return String.fromCharCode(oppCharCode).toLowerCase();
        } else {
            return String.fromCharCode(oppCharCode);
        }
    }
    
    for (start; start <= end; start++) {
        list.push({
            "alphabetNumber": letterNum,
            "letter": String.fromCharCode(start),
            "charCode": start,
            "oppositeCharCode": (end + 1) - letterNum++
        });
    }

    return chars.map(function(char) {
        return opposite(char);
    }).join("");
}