Test file structure, shared fixtures, CI strategies, test debt.

Test Architecture — Organising Tests for Long-term Maintainability

Test architecture determines how maintainable your test suite is as it grows. Key decisions: where test files live (co-located with source vs separate directory), how shared setup is structured (fixtures, factories, builders), how CI runs tests (parallel workers, caching), and how to prevent test debt accumulation. A well-architected test suite is fast, isolated, and has minimal duplication in setup logic.

Interactive Pipeline Flow

UnderstandImplementOptimise

📁 Structure — co-locate files

File co-location: user.service.ts and user.service.test.ts in the same directory. Easier to find, easier to delete together. Alternative: __tests__ directory mirrors src structure.

Architecture Diagrams

Test data factories prevent brittle hardcoded data.

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Key Design Decisions

🏭Factory functions prevent brittle test setup

When you hardcode test data in every test file and add a new required field to your model, every test breaks. Factory functions with defaults mean only the factory needs updating.

📊Coverage gate in CI prevents test debt from accumulating

Without a coverage gate, developers skip writing tests under deadline pressure. Each PR merges with less coverage, and eventually the suite is meaningless. 80% coverage gate keeps debt in check.

🔍Review test code quality, not just test existence

Tests that assert toBeTruthy() or toBeDefined() are false coverage — they pass even if the function returns garbage. Review tests for meaningful assertions. A bad test is worse than no test.

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