test-driven-development

# Test-Driven Development (TDD) ## Overview Write the test first. Watch it fail. Write minimal code to pass. **Core principle:** If you didn't watch the test fail, you don't know if it tests the right thing. **Violating the letter of the rules is violating the spirit of the rules.** ## The Iron Law ``` NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST ``` Write code before the test? Delete it. Start over. **No exceptions:** - Don't keep it as "reference" - Don't "adapt" it while writing tests - Don't look at it - Delete means delete Implement fresh from tests. Period. ## Outside-In Development Start every feature with a high-level test that describes behavior from the user's perspective. Run it, read the failure, and let that failure dictate your next move. As failures push you down the stack, write a new failing test at each layer you drop into — never write code for a layer without a failing test at that layer demanding it. Read `examples/outside-in-testing.md` for a walkthrough of the philosophy and `examples/testing-pyramid.md` for how test types combine into an optimal suite. ### One Change, One Run After every change — writing a test, adding a route, creating a file, implementing a method — run the affected test immediately: ```bash bundle exec rspec spec/features/guest_searches_for_items_spec.rb ``` The failure message is your instruction for what to do next. Don't batch changes and don't guess ahead: one change, one run, read the failure, decide. If you...