Coverage

Learn test coverage measurement.

Coverage

Coverage measures how much of the code your tests exercise. dart test --coverage=coverage produces an LCOV report; genhtml turns it into HTML.

Two types: line coverage (number of executed lines) and branch coverage (how many if/else branches were taken). 100% line does not mean 100% branch.

Coverage is an indicator of test quality but not sufficient alone. Even with high coverage edge cases may be untested. Coverage shows the picture; it does not guarantee quality.

Line vs branch coverage

Line coverage gives the ratio of executed source lines. An if line counts as 'executed' whether the condition is true or false.

Branch coverage tests both arms of an if. For if (x) { a(); } else { b(); } both a() and b() must be called.

Therefore branch coverage is stricter. You can have 90% line coverage but only 60% branch coverage. Both should be tracked.

Exercise Define a CoverageReport

Define a CoverageReport class with final int covered; final int total; CoverageReport(this.covered, this.total); double get percent => total == 0 ? 0 : (covered / total) * 100;. In main create final report = CoverageReport(85, 100); and add print('${report.percent.toStringAsFixed(0)}%');.

Show example solution

class CoverageReport {
  final int covered;
  final int total;
  CoverageReport(this.covered, this.total);
  double get percent => total == 0 ? 0 : (covered / total) * 100;
}

void main() {
  final report = CoverageReport(85, 100);
  print('${report.percent.toStringAsFixed(0)}%');
}

Test yourself

What does dart test --coverage produce?
- LCOV/coverage data
- An executable
- An HTML report
- A test file
Show answer

LCOV/coverage data — dart test --coverage produces LCOV-format data.
Match the coverage parts

--coverage

Flag
lcov.info

Data file
genhtml

HTML generator
100%

Full coverage

Coverage ratio

var lineCoverage = ____;

Show answer

var lineCoverage = 0.85;

Next lesson: Golden tests

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