Most Frequent Subtree Sum LeetCode solution in JavaScript

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var findFrequentTreeSum = function (root) {
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  const hash = new Map();
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  const result = [];
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  let max = 0;
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  const dfs = (node = root) => {
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    if (!node) return 0;
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    const { left, right, val } = node;
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    const sum = val + dfs(left) + dfs(right);
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    const count = hash.get(sum) ?? 0;
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    hash.set(sum, count + 1);
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    max = Math.max(max, count + 1);
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    return sum;
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  };
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  dfs();
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  hash.forEach((value, key) => value === max && result.push(key));
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  return result;
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};