How to run function of parent window when child window closes?

Question

I am calling the javascript window.open() function to load another url in a pop up. Once the users is finished it takes them to the last page that has a link that says close

Answer 1

Along with jerjer answer(top), sometimes in your parent window and child window are not both external or both internal you will see a problem of opener undefined, and you cannot access parent page properties, see window.opener is undefined on Internet Explorer