Jest Code Coverage Essentials Quiz Quiz

Explore key concepts of code coverage in Jest, including configuration, metrics, reporting, and practical usage tips. This quiz helps solidify your understanding of essential code coverage features and techniques in Jest for reliable JavaScript testing.

1. Understanding Coverage Thresholds

   In a project using Jest, which configuration property ensures that tests fail if global code coverage falls below a specified percentage?

   1. coverageThreshold
   2. coverageFilter
   3. thresholdLimit
   4. globalCoverage

   Explanation: The 'coverageThreshold' property is used to specify global and per-file coverage minimums, causing tests to fail if coverage drops below these thresholds. 'coverageFilter' is not a valid configuration option and may be confused with specifying files to include or exclude. 'thresholdLimit' and 'globalCoverage' are not valid Jest configuration properties for controlling coverage thresholds, making them incorrect choices.

2. Metrics Included in Coverage Reports

   Which four main coverage metrics does Jest report when generating code coverage results for your JavaScript files?

   1. Statements, Branches, Functions, Lines
   2. Statements, Classes, Methods, Paths
   3. Functions, Types, Interfaces, Blocks
   4. Modules, Branches, Scopes, Calls

   Explanation: Jest reports coverage on Statements, Branches, Functions, and Lines, which are standard metrics for analyzing how much code is exercised by tests. 'Statements, Classes, Methods, Paths' are not all part of the metrics provided by Jest. The option including 'Types, Interfaces, Blocks' refers to type systems or different testing aspects. 'Modules, Branches, Scopes, Calls' likewise does not reflect the actual metrics reported by Jest.

3. Including Coverage When Running Tests

   Which command would you use to run Jest tests with coverage collection enabled in a JavaScript project?

   1. jest --coverage
   2. jest --collect-coverage
   3. jest --withCoverage
   4. jest test-coverage

   Explanation: The correct flag is '--coverage' when running the Jest command to enable code coverage collection and reporting. 'jest --collect-coverage' is a common mistake, but it's not a valid Jest CLI flag. 'jest --withCoverage' and 'jest test-coverage' are incorrect and not recognized by the Jest command-line interface.

4. Ignoring Files from Coverage

   When you need to exclude a utility function from coverage analysis in Jest, which directive should you add above the function in your source file?

   1. /* istanbul ignore next */
   2. // skip coverage
   3. // jest-ignore-coverage
   4. // exclude from coverage

   Explanation: Jest uses a coverage tool that honors the 'istanbul' comments like '/* istanbul ignore next */' to ignore the next function or statement in coverage reporting. The other options, such as '// skip coverage', '// jest-ignore-coverage', and '// exclude from coverage', are not recognized by the underlying coverage engine, therefore will not exclude code from coverage metrics.

5. Default Coverage Output Directory

   After running Jest with coverage enabled, where are the generated coverage reports stored by default in your project directory?

   1. coverage
   2. output-coverage
   3. test-reports
   4. coverageResults

   Explanation: By default, Jest saves all coverage reports in a directory named 'coverage' at the root of your project. 'output-coverage', 'test-reports', and 'coverageResults' are not the default directories in which Jest places coverage data, though such names sound plausible and might be used in custom setups.