Typo in the code AppSync - onsubscribe code sample

Thanks for the details. I have submitted the document feedback.

For onSubscribe handlers in AWS AppSync Events, the Lambda function should return one of two things:

1. A payload containing an error message if the subscription fails
2. null to indicate a successful subscription

The correct response type for an onSubscribe handler should look like this:

type LambdaAppSyncEventResponse = {
error: string
} | null

If there is indeed a typo in the documentation you're referring to, it's always helpful to report such issues to AWS support or through their documentation feedback channels. AWS regularly updates their documentation based on user feedback to ensure accuracy and clarity.
Sources
Writing event handlers - AWS AppSync Events