How do I get notified when the speech has completed talking?

I'm trying to get this working in my blazor wasm application so I'm using the javascript library.

The problem is, the success message fires before the talking is completed.

function talkToUser(subscriptionKey, region, textToSpeak) {
    var sdk = window.SpeechSDK;
    var speechConfig = sdk.SpeechConfig.fromSubscription(subscriptionKey, region);
    var synthesizer = new sdk.SpeechSynthesizer(speechConfig);
    synthesizer.speakTextAsync(
        textToSpeak,
        function (result) {
            if (result.reason === sdk.ResultReason.SynthesizingAudioCompleted) {
                // success
                window.dotNetObject.invokeMethodAsync('TalkToUserCompleted');
            } else {
                console.error("Speech synthesis canceled, " + result.errorDetails);
            }
            // success
            synthesizer.close();
            synthesizer = undefined;
        },
        function (err) {
            // error
            console.error(err);
            synthesizer.close();
            synthesizer = undefined;
        }
    );
}

Azure-Samples / cognitive-services-speech-sdk

How do I get notified when the speech has completed talking? #2594