[aws-dynamodb] tableStreamArn is undefined

We’re creating a DynamoDB table with a stream. We’re trying to create a lambda.EventSourceMapping, passing in table.tableStreamArn as the eventSourceArn prop like so:

    new lambda.EventSourceMapping(this, props.serviceName + 'eventSourceMapping', {
      eventSourceArn: ddbTable.tableStreamArn,
      target: ddbStreamProcessor,
      startingPosition: lambda.StartingPosition.TRIM_HORIZON
    });

When we try to build (transpiling from TypeScript to JavaScript), we get the following error:

npm run build

> cdk@0.1.0 build /home/ec2-user/scm/app/cdk
> tsc

lib/cdk-stack.ts:63:7 - error TS2322: Type 'string | undefined' is not assignable to type 'string'.
  Type 'undefined' is not assignable to type 'string'.

63       eventSourceArn: ddbTable.tableStreamArn,
         ~~~~~~~~~~~~~~

  node_modules/@aws-cdk/aws-lambda/lib/event-source-mapping.d.ts:9:14
    9     readonly eventSourceArn: string;
                   ~~~~~~~~~~~~~~
    The expected type comes from property 'eventSourceArn' which is declared here on type 'EventSourceMappingProps'


Found 1 error.

Environment

• CDK CLI Version: 1.56.0 (build c1c174d)
• Module Version: 1.56.0
• Node.js Version: v10.16.0
• OS: all
• Language (Version): TypeScript 3.7.2

[RomainMuller]

The tableStreamArn property is optional (meaning it can be undefined) because no stream will exist if the table was created without a stream property. This is perfectly normal and expected.

There are two things you can do to fix your code:

1. If you’re certain the table was created with a stream property, use a ! to inform the typescript compiler that you know the value will not be undefined:

   eventSourceArn: ddbTable.tableStreamArn!,
   

2. If you are obtaining an ITable from outside of your control and cannot guarantee it was created with a stream property, add guard for the case where no stream was created:

   if (ddbTable.tableStreamArn == null) {
      throw new Exception("No table stream is available on this DynamoDB table. It is required."); // Or whatever
   }
   // From here on, the typescript compiler **knows** `ddbTable.tableStreamArn` cannot be `undefined`
   // since an exception would have been thrown if that was the case. It won't complain about the incompatible
   // types in assignment anymore.
   

[skinny85]

Hello!

When I say “directly”, I mean the same way my blog post that you mentioned is passing an S3 Bucket from the ProducingStack to the ConsumingStack.