The problem here is that the call to app.start() is nested inside the completion of require(...), so by the time require(...) completes the caller (anonymous function) has already completed.
Since app.start() is not executed, you won't see the "app" events, so move out the app.start() outside the completion of the require and move it to the anonymous function like this:
(function() {
...
app.oncheckpoint = function (args) {
//...
};
});
app.start();
})();
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