Exists property

I am trying to use the Exists property in javascript. I have an object that I would like to test to see if its still up on the screen or not. I write a loop that says 

 

while( testObject.Exists){

   wait(1)

}

 

Exept when the object is no longer on the screen it says it can't find the object. I get why it can't find the object because it doesn't exist on the screen. How can I test to see if an object is no longer on the screen and how is the Exists property suppose to work?