How test nodejs moment with deleting object?

I work with nodejs and I have one theoretical problem. I don't understand how I can test existence object after deleting. Deleted fields-links to the object, but how can I be sure that deleted all references?

Example:

var x = {};
var y = {};
var z = {};

y.x = x;
z.x = x;

delete y.x; // It's ok!
// But forget delete z.x; - how can I catch this moment through test?