How to capture the close event of an opened window by window.open() in JavaScript?

How to capture the close event of an opened window by window.open() in JavaScript?

This JavaScript code works well for me:

var newwindow = window.open("/newwindow.html");
newwindow.onbeforeunload = function () {
    // processing event here
    alert("new window closed");
}

Note that after the callback function is executed, the newwindow is closed.