https://leetcode.com/problems/longest-palindromic-substring/?tab=Description

longest-palindrome.js

var Substring = function(s, e) {
    this.s = s;
    this.e = e;
    this.l = e - s;
};

var longestPalindromeFromOffset = function(s, i) {
    var j,
      long = new Substring(0, 1);
    for (j = 1; (j <= (s.length - i)) && ((i - j) >= 0); j++) {
        if (s.charAt(i - j) != s.charAt(i + j)) {
            break;
        }
        if ((j * 2 + 1) > long.l) {
            long = new Substring(i - j, i + j + 1);
        }
        continue;
    }
    
    for (j = 0; (j <= (s.length - i)) && ((i - j) >= 0); j++) {
        if ((j > (s.length - i)) || (s.charAt(i - j) != s.charAt(i + j + 1))) {
            break;
        }
        if ((j * 2 + 2) > long.l) {
            long = new Substring(i - j, i + j + 2);
        }
    }
    return long;
};

var longestPalindrome = function(s) {
    var i,
      j,
      long = new Substring(0, 1),
      mid = Math.ceil(s.length / 2),
      result;
    for (i = 0; i <= mid; i++) {
        if ((((s.length - mid - i) * 2 + 1) < long.l) && (((mid - i) * 2) < long.l)) {
            break;
        }
        if (((mid + i) < s.length) && (((s.length - mid - i) * 2 + 1) > long.l)) {
            result = longestPalindromeFromOffset(s, mid + i);
            if (result.l > long.l) {
                long = result;
            }
        }
        if (((mid - i - 1) >= 0) && (((mid - i) * 2) > long.l)) {
            result = longestPalindromeFromOffset(s, mid - i - 1);
            if (result.l > long.l) {
                long = result;
            }
        }
    }
    return s.substring(long.s, long.e);
};