Incremental Code Coverage checks with Jest

Code coverage is an essential metric that can help you gauge the quality of your test suite. It represents the percentage of your code that is executed during the execution of your tests. The higher the coverage, the more confident you can be that your code is working as expected.

Jest is a popular JavaScript testing framework that comes with built-in code coverage reporting. By default, Jest will generate a coverage report for your entire codebase every time you run your tests. However, in some cases, you may want to focus your coverage analysis on only the code that has changed since the last test run. In this article, we'll explore how to run incremental code coverage checks with Jest and why it's important to do so.

Why is Code Coverage Important?

Code coverage is important because it can help you identify parts of your code that are not being tested adequately. If a piece of code is not covered by your tests, there is no guarantee that it works as intended. By increasing your coverage, you can increase your confidence in the quality of your code.

Code coverage is also useful for identifying areas of your codebase that are more error-prone than others. If a particular module or function has a low coverage percentage, it may be a good candidate for additional testing or refactoring to improve its reliability.

Finally, code coverage is a key metric for ensuring that your code meets certain quality standards. Many software development organizations have established minimum code coverage thresholds that must be met before code can be released to production. By tracking code coverage, you can ensure that your code meets these standards and avoid releasing code that is not adequately tested.

What is Incremental Code Coverage Checking?

Incremental code coverage checking is the process of analyzing only the code that has changed since the last test run. By focusing on only the changed code, you can save time and resources by avoiding unnecessary analysis of code that has already been tested.

Incremental code coverage checking is particularly useful in large codebases or projects with long test suites. By avoiding unnecessary analysis of code that has already been tested, you can speed up your test runs and get faster feedback on your code changes.

How to Run Incremental Code Coverage Checks with Jest

Jest provides several options for running incremental code coverage checks. In this section, we'll explore two methods for achieving this.

Method 1: Running Tests on Changed Files Only

The simplest way to run incremental code coverage checks with Jest is to run tests only on the files that have changed since the last test run. You can do this by using the --onlyChanged option with the jest command. For example:


jest --coverage --onlyChanged

This command will run tests only on the files that have changed since the last test run and generate a coverage report for those files only.

Method 2: Running Coverage on Changed Files Only

Another option for running incremental code coverage checks with Jest is to generate a coverage report only for the files that have changed since the last test run. You can do this by using the --lastCommit option with the jest command. For example:


jest --coverage --lastCommit

This command will generate a coverage report for the files that have changed since the last commit. This can be useful if you want to see how the coverage of the changed code has improved since the last commit.

Setting Coverage Thresholds with Jest

In addition to running incremental code coverage checks, Jest also allows you to set coverage thresholds for your code. Coverage thresholds specify the minimum percentage of code that must be covered by your tests in order for your tests to pass. If the coverage falls below the specified threshold, Jest will fail your tests and report the areas of your code that need additional testing.

You can set coverage thresholds using the coverageThreshold configuration option in your jest.config.js file. For example:


module.exports = {
  // ...
  coverageThreshold: {
    global: {
      statements: 80,
      branches: 80,
      functions: 80,
      lines: 80
    }
  }
};

In this example, we've set coverage thresholds of 80% for statements, branches, functions, and lines. This means that if any of these coverage percentages falls below 80%, Jest will fail the tests and report the areas of the code that need additional testing.

You can also set coverage thresholds on a per-file basis using regular expressions. For example:


module.exports = {
  // ...
  coverageThreshold: {
    './src/components/': {
      statements: 90,
      branches: 90,
      functions: 90,
      lines: 90
    }
  }
};

Why is Setting Coverage Thresholds Important?

Setting coverage thresholds is important because it helps ensure that your code is adequately tested before it is released to production. By setting minimum coverage thresholds, you can establish a baseline for the quality of your code and ensure that all new code meets the same quality standards.

Coverage thresholds also help ensure that your code is reliable and free of bugs. If your coverage falls below the specified threshold, it's a sign that there may be areas of your code that are not adequately tested and could contain bugs. By failing tests when the coverage falls below the threshold, you can ensure that these areas are identified and addressed before they cause problems in production.

Conclusion

Code coverage is an important metric for gauging the quality of your tests and ensuring that your code meets certain quality standards. Jest provides several options for running incremental code coverage checks, including running tests only on changed files and generating coverage reports only for changed files.

In addition to running incremental code coverage checks, Jest also allows you to set coverage thresholds for your code. Coverage thresholds help ensure that your code is adequately tested and free of bugs before it is released to production.

By using Jest's code coverage features and setting coverage thresholds, you can improve the quality and reliability of your code, speed up your test runs, and ensure that your code meets the same quality standards across your entire codebase.

Disclaimer: This article was written with help of ChatGPT. But the code snippets have been verified and known to work as of this writing.