Find Palindrome With Fixed Length LeetCode solution in JavaScript

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var kthPalindrome = function (queries, intLength) {
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  let output = [];
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  // 1. We use FIRST 2 digits to create palindromes: Math.floor((3+1)/2)=2
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  let digit = Math.floor((intLength + 1) / 2);
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  for (let i = 0; i < queries.length; i++) {
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    // 2A. Get Nth 2-digits numbers: 10*(2-1)-1+[5,98]=[14,107]
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    let helper = 10 ** (digit - 1) - 1 + queries[i];
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    // 2B. Digits checking: 107>=100, which is INVALID
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    if (helper >= 10 ** digit) {
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      output.push(-1);
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    } else {
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      let m = intLength - digit;
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      // 3A. We still need m digits for REFLECTION: 14=>["1","4"]=>"41"
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      let add = helper.toString().substr(0, m).split("").reverse().join("");
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      // 3B. Multiply 10**m for reversed digits: 14=>1400=>1441
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      helper = helper * 10 ** m + add * 1;
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      output.push(helper);
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    }
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  }
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  return output; // [1441,-1]
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};