Popping prototypes off a stack in javascript results in undefined

I'm trying to demonstrate some search algorithms on a webpage. DFS, BFS, and A*. The end goal is to have the user click a grid box and watch the search function light up the grid tiles as it searches them. I'm having issues getting my data structures to work properly in Javascript though. So this is my Stack prototype.

function Stack()
{
    this._size = 0;
    this._stack = {};
}
Stack.prototype.push = function(node)
{
    var size = this._size++;
    this._stack[size] = node;
}
Stack.prototype.pop = function()
{
    var size = this._size;
    if(size > 0)
    {
        var popped = this._stack[size];
        delete this._stack[size];
        this._size--;
        return popped;
    }
}
Stack.prototype.isEmpty = function()
{
    var size = this._size;
    return size <= 0;
}
Stack.prototype.stringify = function()
{
    var size = this._size
    var str = "Stack:"+ this._size + ":{";
    if(size > 0)
    {
        for(var i = 0; i < size; i++)
        {
            str = str + this._stack[i].stringify();
            if(i+1 < size)
            {
                str = str + ", ";
            }
            else
            {
                str = str + "}";
            }
        }
    }
    else
    {
        str = str + "Empty}";
    }
    return str;
}

Here is the prototype I am pushing and popping to test with

function TILE(x, y, w, h, id)
{
    this._x = x;
    this._y = y;
    this._w = w;
    this._h = h;
    this._id = id;
}
TILE.prototype.X = function(){return this._x;}
TILE.prototype.Y = function(){return this._y;}
TILE.prototype.WIDTH = function(){return this._w;}
TILE.prototype.HEIGHT = function(){return this._h;}
TILE.prototype.ID = function(){return this._id;}
TILE.prototype.stringify = function()
{
    var str = "Tile:"+this._id;
    return str;
};

Here is my test function.

function testStack()
{
    var s = new Stack();
    console.log(s.stringify());
    s.push(new TILE(10, 10, 15, 15, 69));
    console.log(s.stringify());
    s.push(new TILE(10, 10, 15, 15, 70));
    console.log(s.stringify());
    var thing = s.pop();
    console.log(s.stringify());
    console.log(thing.stringify());
}

And here is my console output

Stack:0:{Empty}
Stack:1:{Tile:69}
Stack:2:{Tile:69, Tile:70}
Stack:1:{Tile:69}
TypeError: thing is undefined

I'm somewhat new to Javascript. I understand its not really an object oriented language but it looks to me like the stack working properly up until I try to set 'popped' in TILE.js. Can anyone maybe enlighten me on what's happening here? Can Javascript prototypes be used like this?