Construct Binary Tree from Preorder and Postorder Traversal LeetCode solution in JavaScript

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var constructFromPrePost = function (preorder, postorder) {
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  let preIndex = 0;
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  const dfs = (postArr) => {
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    if (postArr.length === 0) return null;
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    const val = preorder[preIndex];
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    const nextVal = preorder[++preIndex];
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    const mid = postArr.indexOf(nextVal);
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    const root = new TreeNode(val);
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    root.left = dfs(postArr.slice(0, mid + 1));
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    root.right = dfs(postArr.slice(mid + 1, postArr.indexOf(val)));
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    return root;
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  };
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  return dfs(postorder);
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};