I use localStorage
The client app stores a string in known location. The server watches that location. When a string shows up, convert it to uppercase, delete the string, and store the new string in another known location.
The client app is watching that location, when a value shows up, it displays it for the user.
There is a limit — the two apps have to originate from the same domain, because only apps from the same domain can share space in localStorage.
The example app is in a GitHub repository.
Download the client and server and drag them into your browser.
Enter a string in the client. Click the Submit button. See the result.
Seeing is believing
Some people don’t believe this works. It does.
I’ve deployed it in a real-world app, and there have been no support issues.
And with this demo, you can see with your own eyes.