Approval testing is a powerful technique for working with legacy systems. When refactoring, it's important that we have tests to tell us if we've changed behaviour, but sometimes code can be smelly enough that writing unit tests is hard. Approval tests fill this gap. Instead of writing behavioural unit tests, we capture the output of the system - usually as text - and run a diff tool to check for changes in the output as we change the code.

I've previously struggled to set up approval tests with the common tools and libraries, but I realised today that Jest Snapshots solve exactly the same problem, and are easy to use.