This is part 2 of series migrating legacy app. You have setuped the linter and integrated it with CI/CD. Now making changes in code you can be sure typo in variable name will be caught by linter and code style of the appliction became more consistent so making changes just a bit more pleasant in general. But still after making change you are not sure if your changes wont brake previously working functionality. Obvious decision you need testing. But as my experience says you shouldnt start with unit tests, start with small amount of e2e tests.
if we explain very briefly unit tests are tests of small units of codebase. As an example it can be a test of utility function or specific component. Unit test will check if after sending specific arguments we are getting correct respsonse. In practice we might also check what other function were called. And this mean we really care about implementation of this function.
From other side E2E tests doesnt care about utility functions or specific components. You might have broken some unit test by refactoring functionality of some function and moving this logic to another function, but for user nothing has changed. So unit tests might slow down refactoring of the application and doesnt bring much benefits.
When you are writing E2E test first thing that you think should be how consistant and reliable my tests will be. So one test shouldnt interfer with other and one test suite run shouldnt depend from another. But you need to start getting benefits already now, without making changing to CI or introducing some seeding mechanism. So quick answer use already existing data and dont modify or if you modify data in this test run revert them back. Either on success or failure, something like afer, or afterEach.
First set of E2E tests should be simple, cover critical path of your application and regions that you want to refactor without editing data but rather just make sure everything interactable and visible.
This small subset of E2E tests is easy to write and will bring you a lot of benefits.