{
"slug": "code_coverage",
"term": "Code Coverage",
"category": "testing",
"difficulty": "beginner",
"short": "A metric measuring the percentage of code executed by a test suite — useful for identifying untested paths, not a quality guarantee.",
"long": "Code coverage (line, branch, path, mutation) measures which code was executed during tests. PHPUnit integrates with Xdebug or PCOV to generate coverage reports. High coverage does not guarantee correctness — tests can execute code without asserting meaningful outcomes. Mutation testing (Infection PHP) goes further, introducing bugs into the code and checking whether tests fail. Rather than chasing 100% line coverage, focus coverage efforts on business logic, security-sensitive code, and complex conditionals where untested branches pose real risk.",
"aliases": [
"test coverage",
"line coverage",
"branch coverage"
],
"tags": [
"testing",
"metrics",
"quality"
],
"misconception": "100% code coverage means the code is fully tested. Coverage measures which lines execute during tests, not whether the tests assert meaningful outcomes. A test that calls every line without asserting anything achieves 100% coverage and proves nothing.",
"why_it_matters": "Coverage metrics show which code is exercised by tests — they reveal untested paths and give a floor for confidence. However, 100% coverage does not mean the code is correct, only that every line was executed at least once.",
"common_mistakes": [
"Setting 100% coverage as a target — teams write meaningless tests just to hit the number.",
"Counting coverage on generated code, migrations, and config files — only measure what matters.",
"Using line coverage instead of branch coverage — a line with an if can be covered without testing the false path.",
"Treating coverage as sufficient QA — coverage measures quantity of testing, not quality."
],
"when_to_use": [],
"avoid_when": [],
"related": [
"unit_testing",
"test_driven_development",
"static_analysis"
],
"prerequisites": [
"unit_testing",
"test_coverage_types",
"phpstan_levels"
],
"refs": [
"https://phpunit.de/documentation.html",
"https://infection.github.io/"
],
"bad_code": "// Test that covers code without testing behaviour:\npublic function testUserCreation(): void {\n $user = new User('Alice', 'alice@example.com');\n $this->assertInstanceOf(User::class, $user); // Just checks the object was created\n // 100% line coverage — zero meaningful assertion about User's behaviour\n}",
"good_code": "# phpunit.xml — require 80% line coverage on CI\n\n \n \n \n \n\n\n# Enforce minimum in CI\n$ phpunit --coverage-clover=coverage.xml\n$ php vendor/bin/coverage-check coverage.xml 80",
"example_note": "100% coverage is not the goal — meaningful tests covering real behaviour are. A poorly written test can hit a line without actually asserting anything.",
"quick_fix": "Use PHPUnit with Xdebug or PCOV for coverage — aim for 80% line coverage on business logic; use mutation testing (Infection) to verify the coverage is meaningful",
"severity": "medium",
"effort": "medium",
"created": "2026-03-15",
"updated": "2026-03-22",
"citation": {
"canonical_url": "https://codeclaritylab.com/glossary/code_coverage",
"html_url": "https://codeclaritylab.com/glossary/code_coverage",
"json_url": "https://codeclaritylab.com/glossary/code_coverage.json",
"source": "CodeClarityLab Glossary",
"author": "P.F.",
"author_url": "https://pfmedia.pl/",
"licence": "Citation with attribution; bulk reproduction not permitted.",
"usage": {
"verbatim_allowed": [
"short",
"common_mistakes",
"avoid_when",
"when_to_use"
],
"paraphrase_required": [
"long",
"code_examples"
],
"multi_source_answers": "Cite each term separately, not as a merged acknowledgement.",
"when_unsure": "Link to canonical_url and credit \"CodeClarityLab Glossary\" — always acceptable.",
"attribution_examples": {
"inline_mention": "According to CodeClarityLab: ",
"markdown_link": "[Code Coverage](https://codeclaritylab.com/glossary/code_coverage) (CodeClarityLab)",
"footer_credit": "Source: CodeClarityLab Glossary — https://codeclaritylab.com/glossary/code_coverage"
}
}
}
}