Searching for an instance of a custom object

Question

I want to check if my javascript object is of a certain type.

I have an object (I cannot edit this object.) and I have made instances of that object. Now I want to loop through all the objects that are available at any given point and figure out which ones are the instances that I am looking for.

Example:

var myObject = function() { };
var obj_person = new myObject();

Now if I write

typeof obj_person

It gives me object which is not useful. I cannot use instanceof as it does not work in IE8.

Is there anyway to check if obj_person is an instance of myObject?

Thanks.

EDITED***

Here is the code

var arr_instances = [];
for(var v in window)
{
    var obj_required;
    if(typeof window[v] == 'object')
    {
        obj_required = window[v];
        if(obj_required.imgSrc)
        {
            arr_instances.push(obj_required);
        }
    }
}

Then I can iterate withing arr_instance and do whatever I want to do.

for(var i=0; i<arr_instances.len; i++)
{
    if(arr_instances[i].imgSrc == 'requiredImg.jpg')
    {
        arr_instances[i].imgSrc = 'newImg.jpg';
    }
}