Test Coverage Types

Also Known As

TL;DR

Explanation

Line coverage: percentage of source lines executed by tests — easiest to achieve, easiest to game (execute a line without asserting its output). Branch coverage: percentage of if/else/switch branches taken — more meaningful than line coverage. Mutation testing (Infection PHP): automatically modifies code (change + to -, flip conditions) and checks if tests fail — the mutation score reveals whether tests actually detect changes. 100% line coverage with no assertions = 100% line coverage, 0% mutation score.

Common Misconception

Why It Matters

Common Mistakes

• 100% line coverage as the only quality metric
• Tests that cover code without asserting outcomes
• Testing implementation details instead of observable behaviour
• Not running mutation testing on critical business logic paths

Code Examples

// 100% line coverage, 0% useful assertions:
public function testCalculateTotal(): void {
    $cart = new Cart();
    $cart->add(new Item('Widget', 9.99));
    $total = $cart->calculateTotal();
    // No assertion — executes the code but checks nothing!
    $this->assertTrue(true); // 100% line coverage, 0% mutation score
}

// Meaningful coverage with real assertions:
public function testCalculateTotalAppliesVat(): void {
    $cart = new Cart(new VatCalculator(0.20));
    $cart->add(new Item('Widget', 10.00));
    $this->assertSame(12.00, $cart->calculateTotal(), '20% VAT not applied');
}
// Run mutation testing:
// vendor/bin/infection --min-msi=80 --min-covered-msi=90

References

• ↗ https://infection.github.io/guide/

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