Merge and sort two different javascript list

mergeAndSortTwoList.js

// Definition for singly-linked list:
// function ListNode(x) {
//   this.value = x;
//   this.next = null;
// }

let last = false,
    unified = false;
function mergeTwoLinkedLists(l1, l2, first = true) {
    if(!l1){
        if(!l2){
            return l1;
        }
        return mergeTwoLinkedLists(l2, null);
    }
    if(l1 && !l1.next && l2){
        l1.next = l2;
        l2 = null;
        unified = true;
    }
    
    if(l1 && l1.next) {
        if(l1.value <= l1.next.value) {
            mergeTwoLinkedLists(l1.next, l2, false);
        }
    }
    if(!l2 && l1 && !l1.next){
        last = true;
    }
    
    if(l1 && l1.next && l1.value > l1.next.value) {
        l1.value += l1.next.value;
        l1.next.value = l1.value - l1.next.value;
        l1.value = l1.value - l1.next.value;
    }
    if(first && !last){
        mergeTwoLinkedLists(l1, (unified ? null : l2));
    }
    return l1;
}