It passes all tests

The most important feature for a piece of software is that it behaves as expected. Tests reduce the likelihood of writing buggy code and without them software development becomes an exercise of faith. Although manually checking that everything works still qualifies as testing, the real power resides in automated tests. Writing tests little by little as we progress with a codebase, creates a safety net that is always available and provides us with an immediate feedback. It really is a super power and it gives us peace of mind.

Automated tests is one of the three pillars that took anxiety away from my work as a developer. The other two are continuous deployment and observability.