Finding the longest palindrome in a string

Implement a function that finds the longest palindrome in a given string. For example, in the string "My dad is a racecar athlete", the longest palindrome is "a racecar a". Count whitespaces as valid characters. Other palindromes in the above string include "dad", "ete", " dad " (including whitespace on each side of dad).

String "There was a tattarrattat on the racecar. It made a funny noise, gfedcbabcdefg" should return ' tattarrattat '

var longestPalindrome = function (string) {
  var longest = '';
  for (var i = 0; i < string.length; i++) {
    // quick reference to the current letter in the string
    var currentLetter = string[i];
    // find the last occurance of the letter in the string
    var firstOccurance;
    for (var j = i + 1; j < string.length; j++) {
      if (currentLetter === string[j]) {
        // we found an occurance that matches the current letter
        firstOccurance = j;
        // generate the substring to match
        var subString = string.slice(i, firstOccurance + 1);
        // check if the substring is a palindrom and if it is longer then longest
        if (subString.length > longest.length &&
            subString.toLowerCase() === subString.split('').reverse().join('').toLowerCase()) {
              longest = subString;
        }
      }
    }
  }
  // return the longest palindrome
  return longest;
};

Written on March 6, 2015