linked list

    var DoublyLinkedList;
    
    function Node(data) {
        this.data = data;
        this.previous = null;
        this.next = null;
    }

    function DoublyLinkedList() {
        this.head = null;
        this.tail = null;
        this.numberOfValues = 0;
    }

    DoublyLinkedList.prototype.add = function (data) {
    var node = new Node(data);
    if(!this.head) {
        this.head = node;
        this.tail = node;
    } else {
        node.previous = this.tail;
        this.tail.next = node;
        this.tail = node;
    }
    this.numberOfValues++;
    };
    DoublyLinkedList.prototype.remove = function(data) {
    var current = this.head;
    while(current) {
        if(current.data === data) {
        if(current === this.head && current === this.tail) {
            this.head = null;
            this.tail = null;
        } else if(current === this.head) {
            this.head = this.head.next;
            this.head.previous = null;
        } else if(current === this.tail) {
            this.tail = this.tail.previous;
            this.tail.next = null;
        } else {
            current.previous.next = current.next;
            current.next.previous = current.previous;
        }
        this.numberOfValues--;
        }
        current = current.next;
    }
    };
    DoublyLinkedList.prototype.insertAfter = function(data, toNodeData) {
    var current = this.head;
    while(current) {
        if(current.data === toNodeData) {
        var node = new Node(data);
        if(current === this.tail) {
            this.add(data);
        } else {
            current.next.previous = node;
            node.previous = current;
            node.next = current.next;
            current.next = node;
            this.numberOfValues++;
        }
        }
        current = current.next;
    }
    };
    DoublyLinkedList.prototype.traverse = function(fn) {
    var current = this.head;
    while(current) {
        if(fn) {
        fn(current);
        }
        current = current.next;
    }
    };
    DoublyLinkedList.prototype.traverseReverse = function(fn) {
    var current = this.tail;
    while(current) {
        if(fn) {
        fn(current);
        }
        current = current.previous;
    }
    };
    DoublyLinkedList.prototype.length = function() {
    return this.numberOfValues;
    };
    DoublyLinkedList.prototype.print = function() {
    var string = '';
    var current = this.head;
    while(current) {
        string += current.data + ' ';
        current = current.next;
    }
    console.log(string.trim());
    };

module.exports = DoublyLinkedList;