/// Assume `win` refers to your typical application window (a `BrowserWindow` instance)
win.webContents.on("will-prevent-default", ev => {
    do_something().then(result => {
        if(result == some_value) {
            ev.preventDefault(); /// Does NOT work (page doesn't unload) when called from a callback, i.e. when not called immediately (synchronously) in the event handler.
        }
    });
    /// `ev.preventDefault` would have the documented effect if one placed the call here, e.g. somewhere it would be executed as part of the event handler itself, not a callback.
});