Counting Bits LeetCode solution in JavaScript

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var countBits = function (n) {
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  if (n === 0) return [0];
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  const ans = new Array(n + 1).fill(0);
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  let currRoot = 0;
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  for (let idx = 1; idx <= n; idx++) {
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    if ((idx & (idx - 1)) === 0) {
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      ans[idx] = 1;
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      currRoot = idx;
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      continue;
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    }
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    ans[idx] = 1 + ans[idx - currRoot];
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  }
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  return ans;
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};