Find the Closest Palindrome LeetCode solution in JavaScript

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var nearestPalindromic = function (n) {
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  //special case return immediately
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  if (n.length === 1) {
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    return String(n - 1);
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  }
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  const gen99 = (len) => {
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    return "9".repeat(len);
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  };
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  const gen101 = (len) => {
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    return "1" + "0".repeat(len - 1) + "1";
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  };
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  let case1 = gen99(n.length - 1);
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  let case2 = gen101(n.length - 1);
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  let case3 = gen99(n.length);
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  let case4 = gen101(n.length);
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  let tempPalindrome = "";
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  let tempPalindromeRev = "";
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  let startCode = 0;
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  let endCode = n.length - 1;
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  while (endCode - startCode > 1) {
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    tempPalindrome = tempPalindrome + n[startCode];
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    tempPalindromeRev = n[startCode] + tempPalindromeRev;
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    startCode += 1;
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    endCode -= 1;
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  }
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  const addCenterStr = (add) => {
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    if (n.length % 2 !== 0) {
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      return tempPalindrome + (Number(n[startCode]) + add);
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    } else {
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      return (
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        tempPalindrome +
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        (Number(n[startCode]) + add) +
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        (Number(n[startCode]) + add)
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      );
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    }
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  };
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  let case5 = addCenterStr(0) + tempPalindromeRev;
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  let case6 = addCenterStr(-1) + tempPalindromeRev;
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  let case7 = addCenterStr(1) + tempPalindromeRev;
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  //Find smallest difference ans
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  const list = [case1, case2, case3, case4, case5, case6, case7];
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  let tempDiff = null;
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  let ans = "";
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  for (let i of list) {
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    if (i != n) {
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      let diff = Math.abs(n - i);
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      if (diff == tempDiff && Number(i) < Number(ans)) {
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        ans = i;
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      }
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      if (diff < tempDiff || !tempDiff) {
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        tempDiff = diff;
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        ans = i;
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      }
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    }
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  }
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  return String(ans);
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};