ApplicationController.swift

// delare WKScriptMessageHandler as delegate
// add to WKWebViewConfiguration init:
configuration.userContentController.addScriptMessageHandler(self, name: "log")

// MARK: WKScriptMessageHandler

func userContentController(userContentController: WKUserContentController, didReceiveScriptMessage message: WKScriptMessage) {
    if (message.name == "log") {
        print(message.body)
    }
}

TurbolinksDemo.js

function logObject(obj) {
    window.webkit.messageHandlers.log.postMessage(obj);
}

document.addEventListener('turbolinks:load', function() { logObject( window.location.toString() ) }, false);