Singly Linked List and related operations in javascript

SinglyLinkedList.js

var Node = function (val) {
    this.val = val || null;
    this.next = null;
};

var SinglyLinkedList = function () {
    this.head = null;
    this.tail = null;
};


SinglyLinkedList.prototype.push = function (val) {
    var newNode = new Node(val);
    if (!this.head && !this.tail) {
        this.head = newNode;
        this.tail = newNode;
        return;
    }
    newNode.next = this.head;
    this.head = newNode;
};

SinglyLinkedList.prototype.indexOf = function (index) {
    if (index < 0 || !this.val) {
        return;
    }
    if (index === 0) {
        return this.val;
    }
    if (this.next) {
        return this.next.indexOf(--index);
    }
    return;
};

SinglyLinkedList.prototype.length = function () {
    if (!this.val) {
        return 0;
    }
    var tmplen = 1;
    if (this.next) {
        tmplen = 1 + this.next.length();
    }

    return tmplen;
};

SinglyLinkedList.prototype.toArray = function () {
    if (!this.head) {
        return [];
    }
    console.log(this.head);
    var tmpArray = [],
        tmpList = this.head;
    while (tmpList) {
        tmpArray.push(tmpList.val);
        tmpList = tmpList.next;
    }
    return tmpArray;

};

SinglyLinkedList.prototype.toString = function () {
    if (!this.head) {
        console.log("empty");
        return;
    }
    var tmpNode = this.head,
        tmpList = [];
    while (tmpNode) {
        tmpList.push(tmpNode.val);
        tmpNode = tmpNode.next;
    }
    console.log(tmpList.join("->"));
    return;
};

SinglyLinkedList.prototype.pop = function () {
    if (!this.head) {
        return;
    }
    if (this.head.next) {
        this.head = this.head.next;
    } else {
        this.head = null;
        this.tail = null;
    }
};

var sll = new SinglyLinkedList();
sll.push(6);
sll.push(7);
sll.push(8);
sll.toString();
sll.pop();
sll.pop();
sll.toString();