For a long time, I treated the question of how to test Step Functions as something for “later”. You check the happy path anyway, because you see it while trying things out. The Retry and Catch blocks, however, are a piece of JSON that only really runs for the first time when something goes wrong in production. That is exactly the wrong place to find out that an error name has a typo.

With the TestState API, there is now a tool I would use in every Step Functions project. It lets you test a single state of your state machine in the cloud without deploying it. Since mocks are supported, you do not even need real Lambda functions or an IAM role. In this article, we build the whole thing with a small example: a bike rental with a retry for throttling, a catch for “no bike available” and tests with node:test in TypeScript.

In short

The TestState API runs exactly one state of your Step Functions definition and reports whether it succeeded, would be retried or ends up in a catch.…