Find Duplicate Subtrees LeetCode solution in JavaScript

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var findDuplicateSubtrees = function (root) {
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  const map = new Map(),
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    res = [];
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  dfs(root, map, res);
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  return res;
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};
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function dfs(root, map, res) {
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  if (!root) return "#";
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  const subtree = `${root.val}.${dfs(root.left, map, res)}.${dfs(root.right, map, res)}`;
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  map.set(subtree, (map.get(subtree) || 0) + 1);
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  if (map.get(subtree) === 2) {
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    res.push(root);
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  }
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  return subtree;
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}