Balance a Binary Search Tree LeetCode solution in JavaScript

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var balanceBST = function (root) {
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  let arr = []; //store sorted value in array
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  let InOrder = (node) => {
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    //inorder helper to traverse and store sorted values in array
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    if (!node) return;
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    InOrder(node.left);
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    arr.push(node.val);
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    InOrder(node.right);
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  };
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  InOrder(root);
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  let BalancedFromSortedArray = (arr, start, end) => {
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    //create Balanced tree from sorted array
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    if (start > end) return null;
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    let mid = Math.floor((start + end) / 2);
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    let newNode = new TreeNode(arr[mid]);
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    newNode.left = BalancedFromSortedArray(arr, start, mid - 1);
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    newNode.right = BalancedFromSortedArray(arr, mid + 1, end);
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    return newNode;
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  };
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  let Balanced = BalancedFromSortedArray(arr, 0, arr.length - 1);
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  return Balanced;
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};