EM_ASYNC_JS Uncaught Error: undefined when out of the main

Francois M

I'm trying to use JavaScript with promises using the JSPI option.
I created a test project like this:

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
#include <QObject>
#include <cstdio>
#include <iostream>

#ifdef __EMSCRIPTEN__
#include <emscripten.h>
#endif

using namespace std;

EM_ASYNC_JS(int, do_fetch, (), {
    out("waiting for a fetch");
    const response = await fetch("vtkqml.html");
    out("got the fetch response");
    return 42;
});

class ButtonHandler : public QObject
{
    Q_OBJECT

public:
    ButtonHandler(QObject* parent = nullptr)
        : QObject(parent)
    { }

public slots:
    void onButtonClicked()
    {
        cout << "Calling" << endl;
        do_fetch();
        cout << "Returned from fetch" << endl;
    }
};


QGuiApplication*       g_app     = nullptr;
QQmlApplicationEngine* g_engine  = nullptr;
ButtonHandler*         g_handler = nullptr;

int main(int argc, char* argv[])
{
    g_app = new QGuiApplication(argc, argv);

    g_engine = new QQmlApplicationEngine();

    g_handler = new ButtonHandler();
    g_engine->rootContext()->setContextProperty("buttonHandler", g_handler);

    g_engine->loadFromModule("vtkqml", "Main");

    cout << "Calling" << endl;
    do_fetch();
    cout << "Returned from do_fetch" << endl;

    if (g_engine->rootObjects().isEmpty())
    {
        return -1;
    }

    return 0;
}

You can see that I call the EM_ASYNC_JS function first in main and then in a slot.
As a result, I get the following output:

Calling
waiting for a fetch
got the fetch response
Returned from do_fetch
Calling
Uncaught Error: undefined

It seems that the call only works when it's made from the main function, and I don't understand why.
Does anyone have an idea what might be causing this?