This is an example of passing messages from Elm to JavaScript and backwards.
It sends a hole () or empty Tuple to JavaScript, as a message and retrieves a numeric value.
Inspired by [Trouble Connecting Ports and Subscriptions] question
See more on the matter in the official docs for [JavaScript Interop] ports
Since this example features port module, it is impossible to build it with elm reactor.
You have to explicitly specify the --output=index.js
$ elm make Main.elm --output=index.js