function longest_palindrome(s) {
    const palindromes = new Set();
    const finishedPalindromes = [];

    const getLength = (i, c, isEven) => (i - c) * 2 + (isEven ? 0 : 1);
    const getPaired = (i, c, isEven) => i - (i - c) * 2 + (isEven ? 1 : 0);

    for (let i = 0; i < s.length + 1; i++) {
      for (const p of palindromes) {
        const {isEven, center} = p;
        const probLength = getLength(i, center, isEven);
        if (i >= s.length || s[getPaired(i, center, isEven)] !== s[i]) {
          const length = probLength - 2;
          palindromes.delete(p);
          finishedPalindromes.push({...p, length, text: s.slice(getPaired(i - 1, center, isEven), i) })
        }
      }
      if (i >= s.length) {
        continue;
      }
      if (s[i + 1] === s[i - 1]) {
        palindromes.add({isEven: false, center: i});
      }
      if (s[i] === s[i - 1]) {
        palindromes.add({isEven: true, center: i - 1});
      }
    }

    let longest = s.length === 0 ?
        {center: 0, length: 0, isEven: true, text: ''} :
        {center: 0, length: 1, isEven: false, text: s[0]};
    for (const p of finishedPalindromes) {
      if (p.length > longest.length) {
        longest = p;
      }
    }
    return longest.text;
  }