Middle of the Linked List LeetCode solution in JavaScript

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var middleNode = function (head) {
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  var runner1 = head;
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  var runner2 = head?.next;
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  while (runner1 && runner2) {
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    runner1 = runner1?.next;
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    runner2 = runner2?.next?.next;
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  }
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  return runner1;
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};