Merge In Between Linked Lists LeetCode solution in JavaScript

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var mergeInBetween = function (list1, a, b, list2) {
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  let start = list1;
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  let end = list1;
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  for (let i = 0; i <= b && start != null && end != null; i++) {
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    if (i < a - 1) start = start.next;
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    if (i <= b) end = end.next;
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  }
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  let tail = list2;
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  while (tail.next != null) {
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    tail = tail.next;
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  }
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  start.next = list2;
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  tail.next = end;
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  return list1;
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};