Trouble using Asyncify

[iSchraeder]

Hello, I am trying to create a project using Qt for Webassembly, and in trying to learn how to make calls to JavaScript libraries, I started from an example I found online which is the following snippet of code:

EM_ASYNC_JS(emscripten::EM_VAL, fetch_json, (const char *url), {
  url = UTF8ToString(url);

  let response = await fetch(url);

  let json = await response.json();

  return Emval.toHandle(json);
});

Calling this function in a project compiles fine, but yields the following result, suggesting to me that the error is in the JavaScript itself.

What does it mean that Asyncify is undefined? How may I best make calls like this from Qt?
Thank you

[lorn.potter]

Did you configure Qt with -device-option QT_EMSCRIPTEN_ASYNCIFY=1 ?