Ensure error classes are exported

[julienvincent]

Fixes #982

The KafkaJS error classes currently aren't exported which makes it hard to distinguish between application errors and kafka errors, especially while developing utils/libraries on top of KafkaJS.

This is especially confusing as the TS types included do export the error classes meaning these errors are only discovered at runtime.

[Nevon]

Damn. I think it was a mistake to export the error classes from TS to begin with. We should probably have exported error interfaces instead, because there's no situation in which you as a consumer of KafkaJS should be instantiating those errors yourself. You just need to know what fields are publicly available on the error instances that we throw, and do figure out which error you received in case of a union, which we could do by exposing a type field on the error interface instead.

But unfortunately this was done in 1.15.0, so it's a public API now :(

[julienvincent]

@Nevon

because there's no situation in which you as a consumer of KafkaJS should be instantiating those errors yourself

In our case It's not that we want to instantiate the error, but more that we want to do instanceof checks on it. As an example: We have built an eachBatch util which adds some additional logic like retries, tracing, metrics, and error reporting - but we want to fall through when a KafkaJS error is encountered (which might be encountered when calling methods on a batch like heartbeat)

This looks something like:

const eachBatchUtil = (handler: kafkajs.EachBatchHandler): kafkajs.EachBatchHandler => {
  return async (payload) => {
    try {
      await handler(payload)
    } catch (err) {
      if (err instanceof kafkajs.KafkaJSError) {
        throw err;
      }

      // .. attempt retries
    }
  }
}

consumer.run({
  eachBatch: eachBatchUtil(async (payload) => { ... })
})

[Nevon]

Yeah, that's why I mentioned:

figure out which error you received in case of a union, which we could do by exposing a type field on the error interface instead.

We don't even do instanceof comparisons inside KafkaJS itself. Instead, each error class has a static type property that we use to determine which type of error we've received.

[julienvincent]

Oh, gotcha. :)