Automated tests prevent regressions when upgrading

With Drupal 9 almost being end-of-life and unsupported, I've recently been doing a lot of Drupal 9 to 10 upgrade work.

As it's a major version update, there are breaking changes that mean you may need to change your code.

Here's an example of an error caused by a breaking change:

Drupal\Core\Entity\Query\QueryException: Entity queries must explicitly set whether the query should be access checked or not. See Drupal\Core\Entity\Query\QueryInterface::accessCheck().

To fix this, you need to call the accessCheck() method before executing a query and specify whether it should be checked.

But how do you know if it should be checked?

If you have automated tests that pass beforehand and still pass afterwards, you picked the correct option.

If the test passes before and fails afterwards, you found the bug and can fix it immediately to ensure the functionality works as before.