developer-test-driven-development

Write a failing regression test that reproduces the bug first, then write the minimal production code to make it pass, and refactor while keeping the test green. This Red-Green-Refactor cycle ensures the fix is verified and the bug cannot silently return.

Mutation testing tools like Stryker, cargo-mutants, and PIT inject deliberate faults into green builds to verify tests actually fail when bugs are introduced. A high mutant kill rate proves assertion strength, whereas coverage alone cannot prove tests are meaningful.

Yes, Gherkin scenarios written as Given/When/Then translate directly into executable feature and integration tests. This keeps technical test suites mapped one-to-one with business acceptance criteria.

Use property-based testing or fuzzing when verifying invariants that should hold across many inputs, such as parser round-trips. Tools like Hypothesis, Fast-Check, or QuickCheck generate thousands of randomized inputs without prescriptive example lists.

Code must stay under cyclomatic complexity caps (around 10 per function), meet branch coverage thresholds near 85 percent on core logic, pass linters and dependency audits with zero critical findings, and achieve a high mutant kill rate.

TDD guidance should not be the primary rule set for non-engineering tasks like product positioning, conversion copy, or visual design review. It fits code quality, test design, and performance engineering work.