Merging two sorted linked list in JavaScript

So I was trying to basically sort and merge two linked list using Mergesort.

So, this is the code that creates and iterates a linkedlist in JavaScript which I guess is working fine.

function Node(data) {
    this._data = data;
    this.next = null;
}

function LinkedList(_listHead) {
    this.head = _listHead || null;
}

LinkedList.prototype.insert = function(val) {
    var head = this.head;
    if(head === null) {
        this.head = new Node(val);
    }else {
        var currentNode = head;
        while(currentNode.next !== null) {
            currentNode = currentNode.next;
        }
        currentNode.next = new Node(val);       
    }
}

LinkedList.prototype.iterate = function() {
    var head = this.head;
    if(head === null) {
        return null;
    }else {
        var currentNode = head;
        while(currentNode !== null) {
            console.log(currentNode._data);
            currentNode = currentNode.next;
        }
    }
}

But my merge sort algo is not working. What am I doing wrong?

LinkedList.prototype.mergeSort = function() {
    var count = 0;
    if(this.head === null) {
        return null;
    }
    else if(this.head.next === null) {
        return this.head;
    }

    var currentNode = this.head;
    while(currentNode !== null) {
        count++;
        currentNode = currentNode.next;
    }
    var mid = Math.floor(count/2);
    var count2 = 0;
    var rightPointer = this.head;
    while(count2 < mid) {
        rightPointer = rightPointer.next;
        count2++;
    }
    // Assign both the halves
    var rightList = new LinkedList(rightPointer.next);
    var leftList = new LinkedList(null);
    return new merge(this.mergeSort(leftList),this.mergeSort(rightList));
}

LinkedList.prototype.merge = function(leftList,rightList) {
    var resultList = new LinkedList();
    var resultPointer = resultList.head;
    var leftPointer = leftList.head;
    var rightPointer = rightList.head;
    while(leftPointer !== null && rightPointer !== null) {
        var tempNode = null;
        if(leftPointer._data < rightPointer._data) {
            tempNode = leftPointer._data;
            leftPointer = leftPointer.next;
        }else {
            tempNode = rightPointer._data;
            rightPointer = resultPointer.next;
        }
        if(resultList.head === null) {
            resultList.head = new Node(tempNode);
            resultPointer = resultList.head;
        }else {
            resultPointer.next = new Node(tempNode);
            resultPointer = resultPointer.next;
        }
    }
    // Iterate the rest of the lists
    if(leftPointer.next === null) {
        while(rightPointer !== null) {
            resultPointer.next = rightPointer.next;
            resultPointer = resultPointer.next;
        }
    }else {
        while(leftPointer !== null) {
            resultPointer.next = leftPointer.next;
            resultPointer = resultPointer.next;
        }
    }

    return resultList.head;

}

Please explain.