detects whether a string is a palindrome

palindrome.js

function isPalindrome (str) {
    if (typeof str !== 'string') {
        return false;
    }
    var n,
        letters = str.split(''),
        len = letters.length - 1;
    for (n = 0; n <= len; n++) {
        if (n !== len - n && letters[n] !== letters[len - n]) {
            return false;
        }
    }
    return true;
}