Quoting from that page (to make the answer here more specific for those who may read it in future): "You can interact with JavaScript executing on the page by using postMessage() and connecting to the messageReceived() signal. JavaScript can communicate back by using the "navigator.cascades.postMessage()" function and storing a message handler in the "navigator.cascades.onmessage" property."
Note: You may discover as I did that these two interfaces are a bit limited in what you can send through them. They even appear to have different limitations. For example, in one of the directions, you can't send anything that contains embedded double-quotation marks, as I recall!
What you can do, if you don't want to be limited to simple strings or similarly primitive data, is encode the data before sending, and decode it upon receiving. There are routines that work on both sides for doing this.
In the WebView, you can use the global (i.e. on the "window" object) btoa() and atob() routines to encode to base64 before sending, or decode received messages that were base64-encoded in the QML side.
In QML, where we don't have the global window object, we do have the global Qt object, which has atob() and btoa() routines to match.
If you'd like to send "rich" data, you can also use JSON.stringify() on either side, to serialize more complex JavaScript objects (or arrays) to strings prior to the atob() encoding, then on the receiving side use btoa() followed by JSON.parse() to get the strings turned back into JavaScript objects.
