With coroutines in Lua, you could write:

    function handleRequest(req)
      data = db.fetch("user")
      return render("template.html", {user=data})

And if db.fetch and render (both made up functions) are written using the coroutine paradigm, your code would automatically be evented and non-blocking, no need to use libraries to handle your callbacks, and your code has the same style it does if it were blocking.

In db.fetch it uses the "yield" keyword that would tell Lua it is about to do an async operation it should wait till it finish before continue running Lua code, meanwhile run something else instead, making it non-blocking and you can run many of these Lua programs at once on a single thread, all of them concurrently.