// source https://jsbin.com/cidipo

LinkList

function LinkList(){
    var _first = null;
    var _last = null;
    var _length = 0;

    this.add = function(element){
        if(_length === 0){
            _last = _first = {element: element, prev:null, next:null};
        } else {
            _last = _last.next = {element: element, prev: _last, next:null};
        }
        return ++_length;
    };

    this.pop = function(){
        var listItem = _first;
        if(listItem === null){
            return null;
        }
        _first = _first.next;
        if(_first) {
            _first.prev = null;
        }
        _length--;
        return listItem.element;
    };

    var selectAt = function(index) {
        if(index < 0 || index >= _length){
            return null;
        }
        var steps;
        var pointer;

        if(index < _length / 2){
            steps = index;
            pointer = _first;
            while(steps > 0 && pointer !== null){
                pointer = pointer.next;
                steps--;
            }
        } else {
            steps = _length - index - 1;
            pointer = _last;
            while(steps > 0 && pointer !== null){
                pointer = pointer.prev;
                steps--;
            }
        }
        return pointer;
    };

    this.getElementByIndex = function(index){
        var selection = selectAt(index);
        return selection !== null ? selection.element : null;
    };

    this.getObjectByIndex = function(index){
        return selectAt(index);
    };

    this.recount = function(){
        var length = 0;
        var pointer = _first;
        while(pointer !== null){
            length++;
            pointer = pointer.next;
        }
        return length;
    };


    Object.defineProperty(this, 'length', {
        get: function(){
            return _length;
        }
    });

    Object.defineProperty(this, 'firstObject', {
        get: function(){
            return _first;
        }
    });

    Object.defineProperty(this, 'lastObject', {
        get: function(){
            return _last;
        }
    });
}

// TESTS //
console.log("Tests");
function test(name, expression){
    var result = expression ? "Pass" : "Fail";
    if(!expression && console.error){
        console.error(name + ": " + result);
    } else {
        console.log(name + ": " + result);
    }
}

linkList = new LinkList();
linkList.add(30);
test("Pop the 30", linkList.pop() === 30);
test("Pop null", linkList.pop() === null);
linkList.add(20);
linkList.add(10);
test("Pop the 20", linkList.pop() === 20);
test("First Prev Null", linkList.firstObject.prev === null);
test("Pop the 10", linkList.pop() === 10);

for(var i=0; i<10; i++)
{
    linkList.add(i);
}

test("index 4 is 4", linkList.getElementByIndex(4) === 4);
test("index 6 is 6", linkList.getElementByIndex(6) === 6);
test("object 6 is 4.next.next", linkList.getObjectByIndex(6) === linkList.getObjectByIndex(4).next.next);
test("object 4 is 6.prev.prev", linkList.getObjectByIndex(4) === linkList.getObjectByIndex(6).prev.prev);
test("last is 9", linkList.getObjectByIndex(0) === linkList.firstObject);
test("first is 0", linkList.getObjectByIndex(9) === linkList.lastObject);