Coverage measurement

Learn to measure and read test coverage.

Coverage measurement

Coverage measures what percentage of source code your tests touch. Vitest and Jest use c8 or istanbul to report line, branch and function percentages.

You set thresholds in config: below lines: 80 the build fails. This prevents tests from eroding over time.

Coverage is a good indicator but not a quality guarantee; you manually check that important branches are covered.

Thresholds and quality

When you set branches: 70, branch coverage below 70 percent ends the run with an error code. CI catches it and stops the build.

But 100 percent coverage does not catch every bug; a test may call a branch without checking the expected result. So you write meaningful expect assertions.

Per-file thresholds are also possible; a newly added file cannot merge with low coverage.

Exercise Write the formatPrice test

Write the contents of utils.test.jsx. Import formatPrice. Inside test('formats price', () => {}) assert expect(formatPrice(120)).toBe('120 USD') and expect(formatPrice(0)).toBe('0 USD').

Show example solution

import { formatPrice } from './utils';

test('formats price', () => {
  expect(formatPrice(120)).toBe('120 USD');
  expect(formatPrice(0)).toBe('0 USD');
});

Test yourself

Match coverage types

lines

Executed line ratio
branches

Branch coverage
functions

Called functions
thresholds

Lower bound
What does 100 percent coverage mean?
- No bugs remain
- No more tests are needed
- All lines and branches are called, no quality guarantee
- The build will finish in a minute
Show answer

All lines and branches are called, no quality guarantee — 100 percent coverage is not a quality guarantee.

Fill in the blank: set coverage thresholds

export default {
  test: {
    coverage: {
      thresholds: {
        lines: ____,
        branches: 70
      }
    }
  }
};

Show answer

export default {
  test: {
    coverage: {
      thresholds: {
        lines: 80,
        branches: 70
      }
    }
  }
};

Next lesson: Summary - Advanced testing

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