var head value HEAD next undefined prev undefined function insertAfter

var head = {
	value: 'HEAD',
	next: undefined,
	prev: undefined
};

function insertAfter(where, val) {
	var what = {
		value: val,
		next: undefined,
		prev: undefined
	}
	var nxt = where.next;
	where.next = what;
	what.next = nxt;
	if (nxt !== undefined) {
		nxt.prev = what;
	}
	what.prev = where;
	return what;
}

function deleteNode(node) {
	if (node.prev !== undefined) {
		node.prev.next = node.next;
	}
	if (node.next !== undefined) {
		node.next.prev = node.prev;
	}
}

function search(val) {
	var current = head;
	while (current.next !== undefined) {
		if (current.value == val) {
			return current;
		}
		current = current.next;
	}
	console.log("Not found");
}

function printList() {
    var out = '';
	var current = head;
	while (current !== undefined) {
		out += current.value + ' <-> ';
		current = current.next;
	}
	console.log(out);
}

printList();
var helloNode = insertAfter(head, 'hello');
printList();
var worldNode = insertAfter(helloNode, 'world');
printList();
var semicolonNode = insertAfter(helloNode, ',');
printList();
var anotherNode = insertAfter(semicolonNode, 'another');
printList();
deleteNode(anotherNode);
printList();