Count Largest Group LeetCode solution in JavaScript

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var countLargestGroup = function (n) {
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  const hash = {};
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  for (let i = 1; i <= n; i++) {
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    const sum = i
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      .toString()
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      .split("")
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      .reduce((r, x) => r + parseInt(x), 0);
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    if (!hash[sum]) {
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      hash[sum] = 0;
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    }
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    hash[sum]++;
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  }
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  return Object.keys(hash)
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    .sort((a, b) => hash[b] - hash[a])
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    .reduce((res, x) => {
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      const prev = res[res.length - 1];
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      if (!prev || prev === hash[x]) {
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        res.push(hash[x]);
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      }
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      return res;
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    }, []).length;
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};