Handling Exceptions in an Async Javascript function

Question

Building a "Process Orchestration Engine" in C++ as a node add-on. From C++ I call various user-supplied Javascript code snippets, according to a lifecycle. Here is a typical "message_in" method;

async message_in(msg) {
    // Do stuff
    await supervisor->send_message(xxx);
    // Do more stuff
}

My problem is that I want to handle exceptions gracefully, and without the user having to add try catch blocks. Currently if an exception happens in the above method , (In Do stuff), the promise gets set to rejected, and Node moans at me for not handling it.

But on the C++ side I can only "call" the JS Method, I can't see a way to add a catch() handler.

I don't particularly want to use a Global Process handler.

Can anyone think of a way to avoid the Node warnings, as they claim that they will shut down the process for this in future releases.

Lukasz Wojciak · Accepted Answer · 2018-09-12 13:18:03Z

1

When a promise is rejected any .catch handlers assigned to that promise are triggered, what you could do:

async function message_in(msg) {
  await (async () => {
    // Do stuff
    await supervisor->send_message(xxx)
    // Do more stuff
  })().catch((e) => {
      return e; // or do sth sensible with the error
  });
}

This does put some bloat into your method, but you can extract that with a function/method decorator (https://www.sitepoint.com/javascript-decorators-what-they-are/)

edited Sep 12, 2018 at 13:18

answered Sep 12, 2018 at 12:49

Lukasz Wojciak

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7 Comments

Greycon Over a year ago

My problem is that I want to deal with an exception that occurs before the await call. So, in the line // Do stuff. That seems to cause node to look for a catch handler attached to the message_in method, and , of course, there isn't one.

Lukasz Wojciak Over a year ago

Ok, got it - I've updated my answer to reflect this, it would be probably best to create a decorator for using this in the long run.

Greycon Over a year ago

It solves my problem, thanks! However what I would really like to return in the case of an error is a Rejected promise. Do you know how I might do that?

Lukasz Wojciak Over a year ago

in the catch statement you can return an explicitly rejected promise, so: return Promise.reject(e).

Greycon Over a year ago

Unfortunately, when I do that, I am back to Node complaining about an UnhandledPromiseRejectedWarning - chicken and egg :-(

|

snek · Accepted Answer · 2018-09-12 14:38:28Z

0

You can attach the handler quite easily:

Local<Value> res;
TryCatch try_catch(isolate);
if (fn->Call(...).ToLocal(&res)) {
  if (res->IsPromise()) {
    res.As<Promise>()->Catch(context, errorHandlerFunction);
  }
} else {
  // check try_catch
}

answered Sep 12, 2018 at 14:38

snek

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1 Comment

Greycon Over a year ago

The Node error appears in the fn->Call processing, so it seems that it's already too late to attach the handler after return from the Call.

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