Circular Permutation in Binary Representation LeetCode solution in JavaScript

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var circularPermutation = function (n, start) {
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  const grayCodes = [];
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  let startIdx = -1;
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  for (let i = 0; i <= 2 ** n - 1; i++) {
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    grayCodes[i] = i ^ (i >> 1);
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    if (grayCodes[i] == start) startIdx = i;
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  }
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  const res = [];
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  for (let i = 0; i <= 2 ** n - 1; i++) {
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    res[i] = grayCodes[startIdx];
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    startIdx++;
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    if (startIdx == grayCodes.length) startIdx = 0;
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  }
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  return res;
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};