WKUIDelegate provides native UI elements instead of webpage. For example, you can display UIAlertController in response to JavaScript confirm function. You need to confirm WKUIDelegate protocol before using it.

WKWebView_Display_UI_panel_for_JavaScript.swift

// confirm : WKUIDelegate protocol

webView.UIDelegate = self

…..

**  javascript popup box test

// let alert_box_url = "http://www.w3schools.com/js/tryit.asp?filename=tryjs_alert"
// let confirm_box_url = "http://www.w3schools.com/js/tryit.asp?filename=tryjs_confirm"

// usage : webView.loadRequest(NSURLRequest(URL: NSURL(string:alert_box_url)!))

    // JS alert box
    func webView(webView: WKWebView, runJavaScriptAlertPanelWithMessage message: String, initiatedByFrame frame: WKFrameInfo, completionHandler: (() -> Void)) {
        
        let alertController = UIAlertController(title: "Notice", message: message, preferredStyle: .Alert)
        alertController.addAction(UIAlertAction(title: "OK", style: .Default, handler: { action in
            completionHandler()
        }))
        self.presentViewController(alertController, animated: true, completion: nil)
    }
    
    // JS confirm box.
    // Display alertView through UIAlertController instead of JS popup
    // You don't need to write confirm logic. It's for the webpage.
    // ref : https://github.com/qmihara/WebKitDemo/blob/master/WebKitDemo/WebViewController.swift
    
    func webView(webView: WKWebView, runJavaScriptConfirmPanelWithMessage message: String, initiatedByFrame frame: WKFrameInfo, completionHandler: (Bool) -> Void) {
        
        #if DEBUG
            
            print("JAVA Script Confirm")
            
        #endif
        
        let alertController = UIAlertController(title: webView.URL?.host, message: message, preferredStyle: .Alert)
        alertController.addAction(UIAlertAction(title: "Cancel", style: .Cancel, handler: { action in
            completionHandler(false)
        }))
        alertController.addAction(UIAlertAction(title: "OK", style: .Default, handler: { action in
            completionHandler(true)
        }))
        self.presentViewController(alertController, animated: true, completion: nil)
    }