Plan, Do, Check, Act
Part twenty-two of a series of posts about automated testing.
The Deming cycle (Plan, Do, Check, Act) is a model for thinking about continuous improvement of processes such as software development.
A crucial insight related to this is that the rate of improvement depends on having feedback loops as small as possible; the quicker you can get information about the quality of the software (in the “Check” phase), the quicker you can “Act” to adjust development in the right direction.
Automated testing is a means of speeding up this feedback (compared to manual testing) - if you can verify that your changes have not introduced regressions, then you can spend more time testing new features, or move on to the next change. This also suggests a reason that fast unit tests are often preferable to slow end-to-end tests.
To get the quickest feedback, you need to be running the test suite frequently - these days it is quite common to run tests on every commit via a continuous integration server like Jenkins.
How far can you take this idea? If the tests are fast enough, you should be able to integrate them with your editor or IDE to be run whenever a file is saved. This improves the experience of test-driven development, and informs you of test failures before code is even committed.
This idea, of speeding up feedback, is very widely applicable - this the same reason that User Acceptance Testing (which could be considered to be another form of manual testing) should be happening before code review! No point bothering other developers for review or testing if it turns out you are building the wrong thing.