Binary Tree Preorder Traversal LeetCode solution in JavaScript

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var preorderTraversal = function (root) {
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  let res = [];
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  const check = (node) => {
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    if (node === null) return;
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    else res.push(node.val);
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    if (node.left !== null) check(node.left);
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    if (node.right !== null) check(node.right);
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  };
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  check(root);
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  return res;
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};