Longest palindrome

longest_palindrome.js

/**
 * @param {string} s
 * @return {string}
 */
var longestPalindrome = function(s) {
    var longest = s[0];
    for(var i = 0; i < s.length;i++){
        palen = '';
        var j = -1;
        var k = -1;
        if (s[i] === s[i + 1]) {
            palen = s[i] + s[i + 1];
            j = i - 1;
            k = i + 2;
            while(j > -1 && k < s.length){
                if (s[j] == s[k]) {
                    palen = s[j] + palen + s[k];
                    j--;
                    k++;
                } else {
                    break;
                }
            }
            if (palen.length > longest.length) {
                longest = palen;
            }
        } 
        if (i > 0 && s[i - 1] === s[i + 1]) {
            palen = s[i - 1] + s[i] + s[i + 1];
            j = i - 2;
            k = i + 2;
            while(j > -1 && k < s.length){
                if (s[j] == s[k]) {
                    palen = s[j] + palen + s[k];
                    j--;
                    k++;
                } else {
                    break;
                }
            }
            if (palen.length > longest.length) {
                longest = palen;
            }
        } 
    }
    return longest;
};

// Test Cases 
console.log(longestPalindrome('a'));
console.log(longestPalindrome('aaaa'));
console.log(longestPalindrome('abba'));
console.log(longestPalindrome('abbd'));
console.log(longestPalindrome('abbba'));
console.log(longestPalindrome('ababa'));
console.log(longestPalindrome('dfklsdfgwerkooiwerpppooopppwerwetmfdgmldfg'));