I wrote a code like the example below to execute javascript code that has no return value. let webView: WKWebView // after load complete let result = await webView.evaluateJavascript("someFunction()") // :0: Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value but when i use method with completion handler manner, it doesn't be crashed. but Xcode enforces me to use 'await' keyword and warning is bordering me await webView.evaluateJavaScript("someFunction()", completionHandler: nil) // warning: Consider using asynchronous alternative function The differnce I found is the different signature. Completion handler version has Optional result type, but async/await version has just Any result type func evaluateJavaScript(_ javaScriptString: String, completionHandler: ((Any?, Error?) -> Void)? = nil) func evaluateJavaScript(_ javaScriptString: String) async throws -> Any my Xcode version is 13.2.1. Hope to fix it soon.