How to find out the ID of the created window?

Question

Browser extensions

Need to execute some code after the close event of the newly created window. How to find out the ID of a window created in this way:

chrome.windows.create({url: "index.html", type: "popup"});

When the created window is closed, display the following message:

chrome.windows.onRemoved.addListener(function(){
alert("Close!");
});