iOS WKWebView not showing javascript alert() dialog

To solve this you need a WKUIDelegate for your web view. It is the duty of the delegate to decide if an alert should be displayed, and in what way. You need to implement this for alert, confirm and text input (prompt).

Here is sample code without any validation of the page url or security features:

- (void)webView:(WKWebView *)webView runJavaScriptAlertPanelWithMessage:(NSString *)message initiatedByFrame:(WKFrameInfo *)frame completionHandler:(void (^)(void))completionHandler{    UIAlertController *alertController = [UIAlertController alertControllerWithTitle:message                                                                             message:nil                                                                      preferredStyle:UIAlertControllerStyleAlert];    [alertController addAction:[UIAlertAction actionWithTitle:@"OK"                                                        style:UIAlertActionStyleCancel                                                      handler:^(UIAlertAction *action) {                                                          completionHandler();                                                      }]];    [self presentViewController:alertController animated:YES completion:^{}];}

More in the Official Documentation

Swift 3 with all 3 optional functions implemented:

func webView(_ webView: WKWebView, runJavaScriptAlertPanelWithMessage message: String, initiatedByFrame frame: WKFrameInfo,             completionHandler: @escaping () -> Void) {    let alertController = UIAlertController(title: nil, message: message, preferredStyle: .actionSheet)    alertController.addAction(UIAlertAction(title: "OK", style: .default, handler: { (action) in        completionHandler()    }))    present(alertController, animated: true, completion: nil)}func webView(_ webView: WKWebView, runJavaScriptConfirmPanelWithMessage message: String, initiatedByFrame frame: WKFrameInfo,             completionHandler: @escaping (Bool) -> Void) {    let alertController = UIAlertController(title: nil, message: message, preferredStyle: .actionSheet)    alertController.addAction(UIAlertAction(title: "OK", style: .default, handler: { (action) in        completionHandler(true)    }))    alertController.addAction(UIAlertAction(title: "Cancel", style: .default, handler: { (action) in        completionHandler(false)    }))    present(alertController, animated: true, completion: nil)}func webView(_ webView: WKWebView, runJavaScriptTextInputPanelWithPrompt prompt: String, defaultText: String?, initiatedByFrame frame: WKFrameInfo,             completionHandler: @escaping (String?) -> Void) {    let alertController = UIAlertController(title: nil, message: prompt, preferredStyle: .actionSheet)    alertController.addTextField { (textField) in        textField.text = defaultText    }    alertController.addAction(UIAlertAction(title: "OK", style: .default, handler: { (action) in        if let text = alertController.textFields?.first?.text {            completionHandler(text)        } else {            completionHandler(defaultText)        }    }))    alertController.addAction(UIAlertAction(title: "Cancel", style: .default, handler: { (action) in        completionHandler(nil)    }))    present(alertController, animated: true, completion: nil)}

Just to expand a bit, WKWebView requires you to show alerts, prompts, and confirms yourself. Do this by becoming a WKUIDelegate:

#import <WebKit/WebKit.h>@interface MyController : UIViewController<WKUIDelegate>

Then assign the delegate:

web.UIDelegate = self;

Then you need to actually implement alert, prompt, and confirm. I create WKWebViewPanelManager.h/m as an easy implementation, so here's what I do:

- (void)webView:(WKWebView *)webView runJavaScriptAlertPanelWithMessage:(NSString *)message initiatedByFrame:(WKFrameInfo *)frame completionHandler:(void (^)(void))completionHandler {    [WKWebViewPanelManager presentAlertOnController:self.view.window.rootViewController title:@"Alert" message:message handler:completionHandler];}- (void)webView:(WKWebView *)webView runJavaScriptConfirmPanelWithMessage:(NSString *)message initiatedByFrame:(WKFrameInfo *)frame completionHandler:(void (^)(BOOL result))completionHandler {    [WKWebViewPanelManager presentConfirmOnController:self.view.window.rootViewController title:@"Confirm" message:message handler:completionHandler];}- (void)webView:(WKWebView *)webView runJavaScriptTextInputPanelWithPrompt:(NSString *)prompt defaultText:(nullable NSString *)defaultText initiatedByFrame:(WKFrameInfo *)frame completionHandler:(void (^)(NSString * __nullable result))completionHandler {    [WKWebViewPanelManager presentPromptOnController:self.view.window.rootViewController title:@"Prompt" message:prompt defaultText:defaultText handler:completionHandler];}

Of course, it's up to you to filter out bad alert/confirm/prompt requests.