windowopenerpostMessage

Window.opener.postMessage refers to the use of the postMessage API to communicate between a window and the window that opened it. In web browsers, a window opened with window.open retains a reference to its opener via the window.opener property. The postMessage method allows sending data from one window to another across origins, using a targetOrigin to restrict where the message is delivered.

How it works: A window (the sender) calls postMessage on a Window object, passing a message and a targetOrigin. When the message is received by the target window, a message event is fired. The receiving window can listen for such events with a message event listener. The event provides the message data, the origin of the sender, and a reference to the sender window (event.source). If the sender is the opener, the recipient can respond using the same mechanism, often by calling event.source.postMessage back to the sender.

From the opened window, messages can be sent back to the opener using window.opener.postMessage(message, targetOrigin). Conversely, the opener can communicate to the opened window either by holding a reference to the opened window or by listening for messages and replying as needed.

Security considerations are central: always verify the origin of incoming messages (event.origin) and specify a strict targetOrigin in postMessage rather than using the wildcard *. Validate and sanitize any data before acting on it. If the opener or the opened window is closed or navigates away, the reference may become null, and messaging may fail.

Common use cases include cross-origin login flows, payment or OAuth popups, and coordinated interactions between a main page and a popup. The mechanism is widely supported in modern browsers and is a standard means of safe cross-document communication.