N-ary Tree Postorder Traversal LeetCode solution in JavaScript

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var postorder = function (root) {
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  const res = [];
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  function post(node) {
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    if (!node) return;
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    for (let child of node.children) {
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      post(child);
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    }
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    res.push(node.val);
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  }
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  post(root);
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  return res;
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};