longest_palindromic_string.js

const longestPalindromicString = (str) => {

  if(str.length < 1 || str == null){
    return '';
  }

  //store longest and if find longer we update
  let longest = '';

  for(let i = 0; i < str.length; i++){
  
    let oddPalindrome = expandFromCenter(str, i, i);

    let evenPalindrome = expandFromCenter(str, i , i+ 1)
    
    if(oddPalindrome.length > longest.length ){
      longest = oddPalindrome;
    }
        if(evenPalindrome.length > longest.length ){
      longest = evenPalindrome
    }

  }

  return longest;
}

function expandFromCenter (s, left, right){
    let count = 0;
    
    while(s[left - count] && s[left - count] == s[right + count]){
        count ++;
    }
    count--;
    
    return s.slice(left - count, right+ count + 1);
}