data structures - Doubly Linked List in javascript -

i building linked list in javascript. dont understand 1 part.

function node(element) {  	this.element = element;  	this.next = null;  	this.previous = null;  }    function llist() {  	this.head = new node("head");  	  	this.find = find;  	this.findlast = findlast;    	this.remove = remove;  	this.insert = insert;  	this.display = display;  	this.dispreverse = dispreverse;  		  }    function find(item) {  	var currnode = this.head;  	while(currnode.element != item) {  		currnode = currnode.next;  	}    	return currnode;  }      function display(list) {  	var currnode = this.head.next;  	while (currnode != null) {  		console.log(currnode.element);  		currnode = currnode.next;  	}  }      function insert(newelement, item) {  	var newnode = new node(newelement);  	var current = this.find(item);  	newnode.next = current.next;  	newnode.previous = current;  	current.next = newnode;      	// why dont need part?      // since new node got inserted, on thoughts,      // next node of current node should point new node previous 1      // current.next.previous = newnode;  	  }    function remove(item) {  	var currnode = this.find(item);  	if (currnode.next != null) {  		currnode.previous.next = currnode.next;  		currnode.next.previous = currnode.previous;  		currnode.next = null;  		currnode.previous = null;  	}  }    function findlast() {  	var currnode = this.head;  	while (currnode.next != null) {  		currnode = currnode.next;  	}    	return currnode;  }    function dispreverse() {    	var currnode = this.head;  	currnode = this.findlast();    	while(currnode.previous != null) {  		console.log(currnode.element);  		currnode = currnode.previous;  	}  }    var cities = new llist();   cities.insert("conway", "head");   cities.insert("russellville", "conway");   cities.insert("carlisle", "russellville");   cities.insert("alma", "carlisle");   cities.display();    cities.remove("carlisle");  cities.display();  cities.dispreverse();      /*  output should this:     conway  russellville  carlisle  alma    conway  russellville  alma    alma  russellville  conway  */

problem insert function!
let's if have b c node already.
and want insert k after b.

currently, next , previous of b c , each.
the previous element of c b.

once put k after b,
a b k c
(1) next element of k c
(2) previous element of k b.
(3) next element of b k
(4) previous element of c k.

on code wrote in insert function, each line of codes below should process upper statements.
(1) newnode.next = current.next;
(2) newnode.previous = current;
(3) current.next = newnode;
(4) current.next.previous = newnode;

but when run whole code including (4), error has occurred.
i don understand why...
without (4) line of codes, works.

is there can me understand this?

you need step 4 before step 3:

current.next.previous = newnode current.next = newnode 

as is, reference of current.next (c) being set newnode (k) before lookup "old" current.next's previous property ( current.next.previous when points b). reference current node changed assign new value. why current.next.previous returning newnode.previous instead of node reference expect.