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How to check if a window was instantiated from a window.open()

Is it possible to check if a window was opened from a window.open() with the new opened window itself. I only want the website I am opening to be display a specific context when opened from a window.open(). It is cross-origin. For example: if the URL is accessed and not opened from a window.open(), I want it to redirect elsewhere. I know this is possible from the parent window, but not sure if it can be done from the child window
Thanks!

You can use window.opener. This will return a reference to the window that opened the current window or null if it is not opened by open().
if (window.opener) {
// Opened from another window
} else {
// Not opened from another window
}

How know if a window can be closed with js

So as it has been discussed elsewhere, a window can be closed by js using window.close() only if it has been opened by a script.
I have a page that offers a button to open a discussion window. The discussion window opens to a new tab with window.open(). The discussion page has a button that calls window.close(), which closes the discussion window and takes you back to previous tab, so you can continue from where you left off.
The problem is that if someone takes a the url directly to the discussion window, the close button does not work.
Is there a way to detect if the window will be closable with window.close(), so I can show the button only if it will work?

You can check to see if window.opener is not null:
When a window is opened from another window, it maintains a reference
to that first window as window.opener. If the current window has no
opener, this method returns NULL. Windows Phone browser does not
support window.opener. It is also not supported in IE if the opener
is in a different security zone.

You can try using the window.opener object, which returns a reference to the window that opened the current window (if it's another window), or NULL if the current window was not opened via JS.
if (window.opener) //Show button

When to use window.opener / window.parent / window.top

In JavaScript when to use window.opener / window.parent / window.top ?

window.opener refers to the window that called window.open( ... ) to open the window from which it's called
window.parent refers to the parent of a window in a <frame> or <iframe>
window.top refers to the top-most window from a window nested in one or more layers of <iframe> sub-windows
Those will be null (or maybe undefined) when they're not relevant to the referring window's situation. ("Referring window" means the window in whose context the JavaScript code is run.)

I think you need to add some context to your question. However, basic information about these things can be found here:
window.opener
https://developer.mozilla.org/en-US/docs/Web/API/Window.opener
I've used window.opener mostly when opening a new window that acted as a dialog which required user input, and needed to pass information back to the main window. However this is restricted by origin policy, so you need to ensure both the content from the dialog and the opener window are loaded from the same origin.
window.parent
https://developer.mozilla.org/en-US/docs/Web/API/Window.parent
I've used this mostly when working with IFrames that need to communicate with the window object that contains them.
window.top
https://developer.mozilla.org/en-US/docs/Web/API/Window.top
This is useful for ensuring you are interacting with the top level browser window. You can use it for preventing another site from iframing your website, among other things.
If you add some more detail to your question, I can supply other more relevant examples.
UPDATE:
There are a few ways you can handle your situation.
You have the following structure:
Main Window
Dialog 1
Dialog 2 Opened By Dialog 1
When Dialog 1 runs the code to open Dialog 2, after creating Dialog 2, have dialog 1 set a property on Dialog 2 that references the Dialog1 opener.
So if "childwindow" is you variable for the dialog 2 window object, and "window" is the variable for the Dialog 1 window object. After opening dialog 2, but before closing dialog 1 make an assignment similar to this:
childwindow.appMainWindow = window.opener
After making the assignment above, close dialog 1.
Then from the code running inside dialog2, you should be able to use
window.appMainWindow to reference the main window, window object.
Hope this helps.

top, parent, opener (as well as window, self, and iframe) are all window objects.
window.opener -> returns the window that opens or launches the current popup window.
window.top -> returns the topmost window, if you're using frames, this is the frameset window, if not using frames, this is the same as window or self.
window.parent -> returns the parent frame of the current frame or iframe. The parent frame may be the frameset window or another frame if you have nested frames. If not using frames, parent is the same as the current window or self

when you are dealing with popups window.opener plays an important role, because we have to deal with fields of parent page as well as child page, when we have to use values on parent page we can use window.opener or we want some data on the child window or popup window at the time of loading then again we can set the values using window.opener

Initiating a connection between two browser windows in Javascript

I understand that if one window opens another that the opened window the return value of .open(path).
I also get that the child window can access the parent through window.opener.
However, in the event that the parent has since been closed, and another window (or tab) from the same domain has been opened, is there any way to grant access from one window to another in a horizontal fashion? Maybe by passing a reference through a cookie or something?
Thanks.

No, sorry. If you don't have a reference to the window there's no way to get it. You can still send messages to another tab, however, via HTML5's localStorage events

If you in the parent have
window.name="myParentWindow"
the child can do
var parentHandle = (window.opener && !window.opener.closed) ? window.opener :
window.open("","myParentWindow");

What's different when a page is opened by window.showModalDialog?

Anyone knows the difference?
The problem I met is that the page stops working correctly when opened by window.showModalDialog

window.showModalDialog vs window.open
Window.open will open up a new window through Javascript, with the URL and other features of the window that u pass as parameters. Here the parent window which opens the new window and the child window are independent windows.
Eg. Below
`window.open('winOpen.htm','name','height=255,width=250,toolbar=no,directories=no,status=no,
linemenubar=no,scrollbars=no,resizable=no');`
Window.showModalDialogue again works smilar to a window.open only diffrence being its a Modal window, It opens up as a new window but doesnt allow the user to access the parent window, unless you explicitly close it.
Here the child window is dependent on the parent window. If you close the parent window the child would also get closed.
window.showModalDialog("xpopupex.htm","name","dialogWidth:255px;dialogHeight:250px");
ShowModalDialogue windows can be used when u want the user to perform a particular action in the new window before he access the parent window again. like login before he can access the parent page..
tryed to make it as simple as possible...hope this help.. ;)