Automated testing
Code that checks other code. Tests run automatically on every proposed change and fail loudly when something breaks.
What it tells you about a developer
Consistent testing in someone's repositories signals engineering maturity and care for future maintainers, including their own future self.
Developers write tests alongside features, so later changes cannot quietly break what already works. The test suite runs automatically on every proposed change. Test files sit in the repository next to the product code. On any open project, the presence and depth of testing is publicly checkable.