Finds length of the longest substring of a string that is a palindrome in Javascript.

longestPalindrome.js

//  Palindromes are the same forwards as in reverse

//  As an example, if the input was “I like racecars that go fast”, 
//  the largest palindrom substring (racecar)
//  would have the length of 7.

var longestPalindrome = function(s){

  let biggest = 0;
  const stringLength = s.length; 

  let substring;
  
  for(let i = 0; i <= stringLength; i++){
    for(let j = i+1; j  <= stringLength; j++){
      substring = s.slice(i,j);
      if ( checkPalindrome(substring) ) {
          if (substring.length > biggest) {
            biggest = substring.length
          }
      }
    }
  } 
  return biggest
}

function checkPalindrome(string){
  var rev_string = string.split('').reverse().join('');
  return (string === rev_string)
}
  
/** Tests -- 
  * Test.assertEquals(longestPalindrome("a"), 1)
  * Test.assertEquals(longestPalindrome("aa"), 2)
  * Test.assertEquals(longestPalindrome("baa"), 2)
  * Test.assertEquals(longestPalindrome("aab"), 2)
  * Test.assertNotEquals(longestPalindrome("zyabyz"), 6, "Are you sure that is a palindrome?")
  * Test.assertEquals(longestPalindrome("baabcd"), 4)
  * Test.assertEquals(longestPalindrome("baablkj12345432133d"), 9) 
  * */