What is Test Coverage?
The percentage of your code that is executed by automated tests. Higher coverage means more of your code is verified to work correctly.
In plain English
Like a security camera system. 100% coverage means every room is monitored. 50% means half your building is unwatched. You don't necessarily need 100%, but you definitely want cameras on the vault (critical features) and entrances (user-facing endpoints).
What the numbers mean
0% — no tests at all (common in AI-built apps). 20-40% — critical paths covered, most code untested. 60-80% — good coverage for most applications. 80%+ — high coverage, diminishing returns above this. The percentage measures lines of code executed during tests, not quality of tests. 100% coverage with weak assertions is worse than 50% with strong ones.
What to cover first
Authentication and authorization. Payment processing. Core business logic. API endpoints that modify data. Any code where a bug would cost money or trust. Don't aim for 100% — aim for 100% of critical paths.
Coverage tools
Vitest and Jest include built-in coverage reporting. Run your tests with --coverage to see which files and lines are covered. Configure coverage thresholds in your CI pipeline to prevent coverage from decreasing over time.
Frequently asked questions
What's a good coverage percentage?
For AI-built apps going to production, aim for 60-80% coverage on server-side code and critical user flows. 100% coverage isn't necessary and often leads to testing implementation details instead of behavior. Focus on testing what matters rather than hitting a number.
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