Question

Could anyone explain to me why the code sample below reports true? I would have assumed that like in C# the instance of Test1 != instance of Test2.

Update: So I think I will go with some unique identifier stored in the base of both Test1 and Test2.

function Test1() { };

function Test2() { };

var test1 = new Test1();

var test2 = new Test2();

var dict = new Array();

dict[test1] = true;

alert(dict[test2]);

Answer 1

Keys in ‘hashtables’ (objects, basically) are always strings. So anything you add will be converted to a string.

new Test1();

returns a instance of Test1. Converted as a string, this is:

"[object Object]"

The same goes for Test2. So in fact, when storing true under the key of new Test1() as a string, you are working with the exact same record as the one by obtaining with the key new Test2() as a string. In other words,

(new Test1()).toString() == (new Test2()).toString();

The actual object is therefore simply:

{
 "[object Object]": true
}

A solution is overwriting .toString() like this:

Test1.prototype.toString = function() { return "Test1" };
Test2.prototype.toString = function() { return "Test2" };

Then dict[test1] will be stored as dict['Test1'] and dict[test2] as dict['Test2'], which allows you to differ between them. Still, manually setting dict['Test1'] would overwrite things. As far as I know, there is no way to assign an object as a key.