function LinkedList(){    var Node = function(element){        this.element = element;        this.next = null;    };    var length = 0;    var head = null;    this.append = function(element){        var node = new Node(element),            current;        if (head == null){            head = node;        } else {            current = head;            while (current.next){                current = current.next;            }            current.next = node;        }        length++;    };    this.insert = function(position, element){        if (position >=0 && position <=length){            var node = new Node(element),                current = head,                previous,                index =0;            if (position === 0){                node.next = current;                head = node;            } else {                while (index++ < position){                    previous = current;                    current = current.next;                }                node.next = current;                previous.next = node;            }            length++;            return true;        } else {            return false;        }    };    this.removeAt = function(position){        if (position > -1 && position < length){            var current = head,                previous,                index = 0;            if (position === 0){                head = current.next;            } else {                while (index++ < position){                    previous = current;                    current = current.next;                }                previous.next = current.next;            }            length--;            return current.element;        } else {            return null;        }    };    this.remove = function(element){        var index = this.indexOf(element);        return this.removeAt(index);    };    this.indexOf = function(element){        var current = head,            index = -1;        while (current){            if (element == current.element){                return index;            }            index++;            current = current.next;        }        return -1;    };    this.isEmpty = function(){        return length === 0;    };    this.size = function () {        return length;    };    this.toString = function(){        var current = head,            string = ‘‘;        while (current){            string += current.element;            current = current.next;        }        return string;    };    this.getHead = function(){        return head;    }    this.print = function(){    };}var list = new LinkedList();list.append(15);list.append(10);console.log(list.toString());function DoublyLinkedList() {    var Node = function(element){        this.element = element;        this.next = null;        this.prev = null;    };    var length = 0;    var head = null;    var tail = null;    this.insert = function(position, element){        if(position >=0 && position <= length){            var node = new Node(element),                current = head,                previous,                index = 0;            if (position === 0){                if (!head){                    head = node;                    tail = node;                } else {                    node.next = current;                    current.prev = node;                    head = node;                }            } else if (position === length){                current = tail;                current.next = node;                node.prev = current;                tail = node;            } else {                while (index++ < position){                    previous = current;                    current = current.next;                }                node.next = current;                previous.next = node;                current.prev = node;                node.prev = previous;            }            length++;            return true;        } else {            return false;        }    }    this.removeAt = function(position){        if (position > -1 && position < length){            var current = head,                previous,                index = 0;            if (position === 0){                head = current.next;                if (length === 1){                    tail = null;                } else {                    head.prev = null;                }            } else if(position == length-1){                current = tail;                tail = current.prev;                tail.next = null;            } else {                while (index++ < position){                    previous = current;                    current = current.next;                }                previous.next = current.next;                current.next.prev = previous;            }            length--;            return current.element;        } else {            return null;        }    }}