doubly linked list

gistfile1.js

var Node = function() {
    this.data = arguments[0];
    this.next = arguments[1] === undefined ? null : arguments[1];
    this.prev = arguments[2] === undefined ? null : arguments[2];
};

var LinkedList = function() {
    this.head = null;
    this.tail = null;
    this.length = 0;
}

LinkedList.prototype.append = function(data) {
    if (this.tail === null) {
        this.tail = this.head = new Node(data);
    } else {
        originalTail = this.tail;
        originalTail.next = this.tail = new Node(data, null, originalTail);
    }
    this.length++;
};

LinkedList.prototype.prepend = function(data) {
    if (this.tail === null) {
        this.tail = this.head = new Node(data);
    } else {
        originalHead = this.head;
        originalHead.prev = this.head = new Node(data, this.head, null);
    }
    this.length++;
};

LinkedList.prototype.pop_front = function() {
    var head = this.head;
    if (!head) return undefined;
    if (!head.next) {
        this.head = null;
        this.tail = null;
    } else {
        this.head = this.head.next;
        this.head.prev = null;
    }
    this.length--;
    return head.data;
}

LinkedList.prototype.pop_back = function() {
    var tail = this.tail;
    if (!tail) return undefined;
    if (!tail.prev) {
        this.head = null;
        this.tail = null;
    } else {
        this.tail = this.tail.prev;
        this.tail.next = null;
    }
    this.length--;
    return tail.data;
};

LinkedList.prototype.popAt = function(index) {
    if (index === 0) {
        return this.pop_front();
    } else if (index === this.length - 1) {
        if (this.length === 0) throw "Error: empty list.";
        return this.pop_back();
    } else {
        node = this.nodeAt(index);
        node.prev.next = node.next;
        node.next.prev = node.prev;
    }
    this.length--;
    return node.data;
};

LinkedList.prototype.nodeAt = function(index) {
    var current;
    if (index === 0) {
        current = this.head;
    } else if (index === this.length - 1) {
        current = this.tail;
    } else if (index > 0) {
        current = this.head;
        for (i=0; i<index; i++) {
            if (current.next === null) { throw "Index is out of range."; };
            current = current.next;
        }
    } else {
        current = this.tail;
        for (i=-1; i>index; i--) {
            if (current.prev === null) { throw "Index is out of range."; };
            current = current.prev;
        }
    }
    return current;
};

LinkedList.prototype.insertAt = function(index, data) {
    if (index === 0) {
        this.prepend(data);
    } else if (index === this.length - 1) {
        this.append(data);
    } else {
        node = this.nodeAt(index);
        newNode = new Node(data, node, node.prev);
        newNode.prev.next = newNode;
        newNode.next.prev = newNode;
    }
    this.length++
};

LinkedList.prototype.delete = function(index) {
    this.popAt(index);
};

LinkedList.prototype.print = function() {
    var output = [];
    var current = this.head;
    while (current != null) {
        output.push(current.data);
        current = current.next;
    }
    console.log(output);
    return output;
};


var ll = new LinkedList();
ll.append("hello");
ll.append("class");
ll.delete(1);
ll.append("world");
ll.prepend("oh!");

ll.print();
// [ "oh!", "hello", "world" ]


var mm = new LinkedList();
mm.append("hello0");
mm.append("hello1");
mm.append("hello2");
mm.append("hello3");
mm.insertAt(2, "hello1.5");
mm.insertAt(-2, "hello1.75");

mm.print();
// [ 'hello0', 'hello1', 'hello1.5', 'hello1.75', 'hello2', 'hello3' ]

console.log(mm.nodeAt(0).data);
// hello0

console.log(mm.nodeAt(-2).data);
// hello2

console.log(mm.popAt(-2));
// hello2

console.log(mm.popAt(0));
// hello0

mm.print();
// [ 'hello1', 'hello1.5', 'hello1.75', 'hello3' ]