Coverage and Its Limits

Coverage shows what ran, not what was checked

Coverage tools report which statements, branches, functions and lines executed during tests. There are two main approaches: istanbul instruments your source code before running it, while v8 uses the coverage data the V8 engine collects natively, which is usually faster and needs no instrumentation step. In Vitest you choose coverage.provider: 'v8' or 'istanbul' and install the matching @vitest/coverage-* package; Jest uses coverageProvider: 'babel' (istanbul-based, the default) or 'v8'. Accuracy of the two has converged in recent releases; check the docs for your version. Remember the limit: a line can be executed by a test that asserts nothing about it. 100% coverage does not mean correct behaviour, and chasing it produces low-value tests. Use coverage to find untested areas, and set modest thresholds to stop it sliding.

Coverage configuration and commands

Vitest config shown; Jest options in the comment.

// vitest.config.ts
import { defineConfig } from 'vitest/config';

export default defineConfig({
  test: {
    coverage: {
      provider: 'v8',                        // or 'istanbul' (npm i -D @vitest/coverage-v8 / -istanbul)
      reporter: ['text', 'html', 'lcov'],
      include: ['src/**/*.{ts,tsx}'],
      exclude: ['src/**/*.test.{ts,tsx}', 'src/test/**', 'src/**/*.d.ts'],
      thresholds: { lines: 80, branches: 70 },   // fail the run if coverage drops below
    },
  },
});

// run:  npx vitest run --coverage
// Jest: collectCoverage / --coverage, coverageProvider: 'v8' | 'babel',
//       coverageThreshold: { global: { lines: 80, branches: 70 } }

Look at uncovered branches, not the percentage

The HTML report highlights branches never taken, such as error paths and empty states. Those are exactly the cases worth a test; the overall percentage is just a trend line.

Quick check: What does 100% line coverage guarantee?

• The code has no bugs
• Every line ran during the tests, not that its behaviour was checked
• Every branch and edge case was asserted
• The UI is accessible