JavaScript Solution
\nvar mergeInBetween = function(list1, a, b, list2) {\n    let start = list1;\n    let end = list1;\n    \n    for (let i = 0; i <= b && start != null && end !=
```\nvar mergeInBetween = function(list1, a, b, list2) {\n    let start = list1;\n    let end = list1;\n    \n    for (let i = 0; i <= b && start != null && end != null; i++) {\n        if (i < a - 1) start = start.next;\n        if (i <= b) end = end.next;\n    }\n    \n    let tail = list2;\n    \n    while (tail.next != null) {\n        tail = tail.next;\n    }\n    \n    start.next = list2;\n    tail.next = end;\n    \n    return list1;\n};\n```
['JavaScript']