Find Latest Group of Size M LeetCode solution in JavaScript

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var findLatestStep = function (arr, m) {
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  if (m === arr.length) return arr.length;
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  let bits = new Array(arr.length + 1).fill(true),
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    pos,
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    flag,
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    i,
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    j;
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  for (i = arr.length - 1, bits[0] = false; i >= 0; i--) {
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    (pos = arr[i]), (bits[pos] = false);
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    for (j = 1, flag = true; flag && j <= m; j++) flag = bits[pos - j];
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    if (flag && !bits[pos - m - 1]) return i;
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    for (j = 1, flag = true; flag && j <= m; j++) flag = bits[pos + j];
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    if (flag && !bits[pos + m + 1]) return i;
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  }
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  return -1;
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};