Longest Univalue Path LeetCode solution in JavaScript

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var longestUnivaluePath = function (root) {
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  let max = 0;
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  const dfs = (node = root, parentVal) => {
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    if (!node) return 0;
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    const { val, left, right } = node;
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    const leftPath = dfs(left, val);
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    const rightPath = dfs(right, val);
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    max = Math.max(max, leftPath + rightPath);
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    return val === parentVal ? Math.max(leftPath, rightPath) + 1 : 0;
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  };
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  dfs();
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  return max;
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};