Convert Sorted List to Binary Search Tree LeetCode solution in JavaScript

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var sortedListToBST = function (head) {
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  if (!head) return null;
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  if (!head.next) return new TreeNode(head.val);
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  let fast = head,
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    slow = head,
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    prev = head;
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  while (fast && fast.next) {
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    prev = slow;
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    slow = slow.next;
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    fast = fast.next.next;
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  }
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  const root = new TreeNode(slow.val);
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  prev.next = null;
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  const newHead = slow.next;
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  slow.next = null;
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  root.left = sortedListToBST(head);
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  root.right = sortedListToBST(newHead);
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  return root;
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};