Longest Palindromic Substring LeetCode solution in JavaScript

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/*
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 * @param {string} s
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 * @return {string}
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 */
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var longestPalindrome = function (s) {
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  let longestP = s[0];
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  let palindrome = "";
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  if (s.length === 1) return s;
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  //the length of the palindrome is odd
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  for (let index = 1; index < s.length - 1; index++) {
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    if (s[index - 1] === s[index + 1]) {
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      palindrome = s[index - 1] + s[index] + s[index + 1];
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      for (let k = 1; index - 1 - k > -1 && index + 1 + k < s.length; k++) {
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        if (s[index - 1 - k] === s[index + 1 + k]) {
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          palindrome = s[index - 1 - k] + palindrome + s[index + 1 + k];
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        } else {
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          break;
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        }
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      }
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    }
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    if (palindrome.length > longestP.length) {
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      longestP = palindrome;
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    }
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    palindrome = "";
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  }
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  //the length of the palindrome is pair
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  for (let index = 0; index < s.length - 1; index++) {
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    if (s[index] === s[index + 1]) {
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      palindrome = s[index] + s[index + 1];
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      for (let k = 1; index - k > -1 && index + 1 + k < s.length; k++) {
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        if (s[index - k] === s[index + 1 + k]) {
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          palindrome = s[index - k] + palindrome + s[index + 1 + k];
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        } else {
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          break;
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        }
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      }
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    }
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    if (palindrome.length > longestP.length) {
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      longestP = palindrome;
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    }
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    palindrome = "";
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  }
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  return longestP;
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};