Blog
Mert Özen's blog posts on software development, backend, frontend, and DevOps.
What Is TDD? Canon TDD, the Test List and the Real Purpose of Red-Green-Refactor
TDD is not writing lots of unit tests but a discipline of growing behavior in small steps while correcting the design all the time: a test list first, then one test from it, red, green, an optional refactor and an updated list. We look at Kent Beck's Canon TDD, how it differs from Uncle Bob's three laws, and how the list for Bookstore's CouponValidator changes over two loops, with real test output.
Red-Green-Refactor in Practice: Fake It, Triangulation, Obvious Implementation and Baby Steps
There are three ways to turn a red test green: obvious implementation when you know the code, fake it when you don't, triangulation when the design is unclear; and every surprising red means a smaller step. Growing CouponValidator live with xUnit v3, we see a banker's rounding surprise, a fake that breaks old tests, verifying a test that was green right away by breaking the code, an Istanbul-time expiry rule with TimeProvider and a small commit on every green, with real output.
Practicing TDD with Katas: String Calculator, Bowling Game and Deliberate Repetition
A kata is practicing the TDD rhythm through deliberate repetition, solving a small problem with a known answer many times; success means step quality, not finishing. We cover coding dojo, randori and ping-pong formats, and solve String Calculator with C# and xUnit v3 Theory and Bowling Game with TypeScript and Vitest, following the test list, the critical decision points and variations that make a kata harder, with real output.
Outside-In TDD: The Double Loop, From Acceptance Test to Domain, and London vs Classical in Practice
Outside-in TDD builds an outer loop with an acceptance test that stays red until the feature is done, and an inner loop of unit tests that grow the pieces to make it pass. We start POST /api/cart/{id}/coupon as a walking skeleton with WebApplicationFactory, discover collaborators with mocks, test the same handler the classical way and compare, with real output, which tests break under a behavior-preserving refactoring.
TDD in Angular: Building a Signal-Based Component Test-First with Vitest
Starting TDD in Angular from a plain TypeScript function instead of the component keeps tests fast and the component thin. With Angular 22 and Vitest we grow cart totals while catching a floating point surprise, a CartSummary component with signal inputs using whenStable and setInput, and an HttpClient service by writing the request first with provideHttpClientTesting; and we see with real output why fakeAsync fails in a zoneless project.
TDD for Bug Fixes and Legacy Code: Red Test First, Characterization and Sprouting New Code
When fixing a bug, TDD starts with a red test that reproduces it at the lowest right level; in legacy code without tests, characterization tests first record today's behavior, and a new rule is grown test-first in a separate sprout and wired in with one line. In an untested CheckoutService we see, with real output, why a characterization test that had recorded the bug turns red after the fix, and how the two kinds of test differ.
The Limits of TDD: The "Is TDD Dead?" Debate, Research, When Not to TDD and TDD with AI Agents
TDD is a trade-off, not a ritual: research shows a small quality gain and mixed productivity results, and suggests the benefit may come from short, steady steps more than from the test-first order. We cover the 2014 "Is TDD Dead?" debate, where TDD falls short (spikes, UI, performance), rules against AI coding agents' tendency to skip and loosen tests, and a CI step that catches it, with real output.