WkWebView JavaScript script not working

My Goal is to send data from iOS device to WkWebView. After researching on how to do this task I get to know that it can be done via executing JavaScript function via WkWebView, therefore, I tried calling just a simple alert function:

@interface UIWebViewController ()  <WKNavigationDelegate, WKUIDelegate>



@end

WKWebView *webView;

-(void)webView:(WKWebView )webView didFinishNavigation:(WKNavigation )navigation{

    NSString* javascriptString = @"alert('Hi');";
    [webView evaluateJavaScript:javascriptString completionHandler:nil];
}
- (void)viewDidLoad {
    [super viewDidLoad];

    WKWebViewConfiguration *theConfiguration = [[WKWebViewConfiguration alloc] init];
    webView = [[WKWebView alloc] initWithFrame:self.view.frame configuration:theConfiguration];
    webView.navigationDelegate = self;   

}

Unfortunately I am not able to see any alert with that code?

Can you let me know what kind of silly mistake I am doing in the above code?