Increasing Order Search Tree LeetCode solution in JavaScript

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var increasingBST = function (root) {
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  let newRoot = new TreeNode(-1);
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  const newTree = newRoot;
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  const dfs = (node) => {
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    if (!node) return null;
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    if (node.left) dfs(node.left);
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    const newNode = new TreeNode(node.val);
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    newRoot.right = newNode;
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    newRoot.left = null;
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    newRoot = newRoot.right;
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    if (node.right) dfs(node.right);
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  };
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  dfs(root);
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  return newTree.right;
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};