Beautiful Array LeetCode solution in JavaScript

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var beautifulArray = function (n) {
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  const EVEN = 1;
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  const ODD = 2;
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  let helper = (arr, flag) => {
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    if (arr.length <= 2) return arr;
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    let even, odd;
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    if (flag == ODD) arr = arr.map((el) => (el + 1) / 2);
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    else if (flag == EVEN) arr = arr.map((el) => el / 2);
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    even = helper(
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      arr.filter((el) => el % 2 === 0),
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      EVEN
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    );
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    odd = helper(
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      arr.filter((el) => el % 2 !== 0),
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      ODD
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    );
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    arr = even.concat(odd);
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    if (flag == ODD) arr = arr.map((el) => el * 2 - 1);
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    else if (flag == EVEN) arr = arr.map((el) => el * 2);
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    return arr;
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  };
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  let arr = new Array(n);
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  for (let i = 0; i < n; i++) arr[i] = i + 1;
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  return helper(arr);
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};