One of the biggest challenges in building zoid and post-robot was figuring out how to get a handle on different windows. A big part of both of these modules is post-messaging between windows and frames, and rendering to different windows.

To get all of that to work across each major browser, quite a few tricks were needed to get a handle on the right window. Getting the window object for the popup or frame you want to reference is the literally the only way you can hope to send a message to it, using the window.postMessage api.

For anyone who’s interested in building cross-window or cross-domain javascript, this reference should be helpful getting a reference to the window you want to message.

Window Group

The first thing to understand is, I can only get a reference to a window if I’m in the same window-group. I’m using that term to mean the collection of windows which includes:

Popup windows that were opened by my window, or a child of my window

Frames that were opened by my window, or a child of my window

Ancestors of my window

So if you’re in a window/tab opened by a user, and you want to get a reference to another window that the user opened (like say they opened another tab), you probably aren’t going to be able to get a handle on it.

That said, here are the windows you can get.

Current Window

This one is easy. window will always refer to the current window.

New Popup Window

If you open a window using window.open , you’ll get a reference to the window you opened.

var newWin = window.open('https://www.google.com', 'windowName');

Existing Popup Window

If someone has opened a popup window with window.open , and you know the window name they used, you can get a handle on that window by calling window.open without an empty string as the url, and with the same name.

var existingWin = window.open('', 'windowName');

This will even work if the popup window was opened from a different frame on the same domain.

In modern browsers, this will even work if the window was opened from a different frame on a different domain. But be warned — this can cause some strange behavior in IE.

Popup Windows as they open

If you’re writing a library and you need to keep a collection of popup windows that are opened by any other library or javascript code, you can monkey patch window.open :

var windows = []; var winOpen = window.open; window.open = function() {

var win = winOpen.apply(this, arguments);

windows.push(win);

return win;

};

Just bear in mind, if anyone opens a popup window before your code loads, you’re not going to catch it, after which the only recourse is to get the window by name.

Window Opener

If you’re inside a popup window, you can get a reference to the parent window by using window.opener .

This property is accessible cross domain, so even if you have a window reference for a window on a different domain, you can still access crossDomainWindow.opener

Frame Parent

If you’re inside a fame, you can get a reference to the parent window by using window.parent .

This property is accessible cross domain, so even if you have a frame reference for a window on a different domain, you can still access crossDomainFrame.parent

Frame Top Window

If you’re in a frame, or a nested iframe, you can get a reference to the top-level window by using window.top .

All Frames in a window

You can get all of the frames that are directly inside a window by accessing window.frames . You can iterate over this by referencing window.frames.length :

for (var i = 0; i < window.frames.length; i++) {

console.log(window.frames[i]);

}

This property is accessible cross domain, so even if you have a popup or frame reference for a window on a different domain, you can still access crossDomainWindow.frames .

Fun fact: window.frames is actually just an alias for window , so window.frames === window , window.frames.length === window.length , and window.frames[0] === window[0] .

Named frames in a window

If your iframe was created with a name, for example:

<iframe name="myIframeName" src="https://www.google.com" />

You can access it by name by referencing window.frames['myIframeName'] or window.frames.myIframeName — or even just window.myIframeName .

These properties are accessible cross domain, so even if you have a popup or frame reference for a window on a different domain, you can still access crossDomainWindow.myIframeName .

Receive a postMessage

If a window has a reference to your window, and that window is feeling kind, it can send a message to you, and you can get a reference to it by referencing event.source . So

In the first window:

window.addEventListener('message', function(event) {

if (event.data === 'identify_me') {

console.log(event.source, 'is saying hello!');

}

});

In the second window:

firstWindow.postMessage('identify_me', '*');

This will work for popups and frames in most modern browsers. The exception to this rule is IE, which steadfastly prevents you from sending post-messages between a window and a popup window.

Iframe Element Window

If you’ve rendered an iframe, like:

<iframe id="myframe" src="https://www.google.com" />

You can get a handle on its window by using document.getElementById('myframe').contentWindow providing the frame has already been inserted into the DOM.

Obviously you can not do this cross-domain.

Same Domain Global

If you’re on the same domain as a another window, you have access to globals on that window. You can use that mechanism to pass around window references. For example

In the first window:

window.winRef = window.open('https://www.google.com', 'windowName');

And in the second window:

console.log(firstWindow.winRef);

Alternatively, a window could pass itself, or another window, to a global function in another window.

In the first window:

function identifyMe(win) {

console.log(win, 'said hello');

}

And in the second window:

window.opener.identifyMe(window);

This could be useful if the opener didn’t get a chance to get a handle on the window as it was being opened.

Chaining

Most of these rules can be chained together. So I could reference somePopup.frames[0].frames[1] or event.source.opener.top , providing I know the correct path to the window I’m looking for.

Impossible Window References

Sometimes it’s not possible to get a reference to the right window. For example:

I’m in IE

I render a cross-domain iframe

That iframe opens a popup window

There’s literally no way for me to get a handle on the popup window from the original parent window. Even if the window wants me to have a reference, it can’t send me a post-message, because of the IE restriction on post-messages between popups.

If you can think of a way to do this, please let me know!

One thing I’d really like is for there to be some browser api to get a reference to any window, from any other window, perhaps by name — like window.getWindowHandle(name) . That would negate a lot of the above hacks.

Others?

If you can think of another method to get a handle on different windows, let me know and I’ll add it here! I think this is an exhaustive list, but I’m very happy to be proven wrong.