I've been thinking about building this for Tcl. Tcl has been available in the browser in various forms for decades, but one thing that makes Tcl uniquely suited to a web application style development is its threading model. In Tcl, you access a thread by sending messages to it, in which a Tcl interpreter inside its event loop processes. My plan was to have a client-side Tcl program in the browser that automatically instantiates a second thread, but the thread is a remote thread running on the server, so you would be able to access the server just as if it were another thread.

Something like (client-side):

    thread::send $::tcl-web::server -async {
        do stuff
    } placeToStoreResult
    ...
    vwait placeToStoreResult
    do stuff with result $placeToStoreResult

You should give Nim a try. It's a Python-like language which can be compiled to C and JavaScript. There is even a SPA framework[1] for it already and the NimForum[2] is written in it.

1 - https://github.com/pragmagic/karax

2 - http://forum.nim-lang.org/

While not exactly that, https://anvil.works/ is kind of heading that way (https://news.ycombinator.com/item?id=15584124)