What are the best practices for using code coverage reports to identify gaps in your unit testing?

1 Understand code coverage metrics

Code coverage reports typically use different metrics to quantify how much of your code is tested by your unit tests. These metrics include line coverage, which is the percentage of lines of code that are executed by your unit tests; branch coverage, which is the percentage of branches (such as if-else statements) that are taken by your unit tests; statement coverage, which is the percentage of statements (such as assignments, function calls, or returns) that are executed by your unit tests; and function coverage, which is the percentage of functions or methods that are called by your unit tests. It's important to understand each metric and how it relates to your testing goals and requirements since none of them can capture all aspects of code quality and test completeness.

2 Set realistic code coverage targets

Code coverage reports can help you set and monitor your code coverage targets, which are the minimum levels of coverage that you want to achieve for your code. Code coverage targets can vary depending on the type, complexity, and criticality of your code, as well as the resources and time available for testing. However, you should avoid setting arbitrary or unrealistic targets, such as 100% coverage for all metrics, without considering the context and value of your code and tests.

Instead, you should set code coverage targets that are aligned with your testing objectives, risk assessment, and quality standards. You should also prioritize the most important and relevant parts of your code, such as the core functionality, the edge cases, and the error handling, and aim for higher coverage for those areas. Additionally, you should review and adjust your code coverage targets periodically, as your code and tests evolve over time.

3 Analyze code coverage reports in detail

Code coverage reports can provide a lot of useful information about your code and tests, but you need to analyze them carefully and critically to make the most of them. Don't rely on the overall or average coverage numbers, as they can be misleading or inadequate for evaluating test quality and effectiveness. Instead, you should take a closer look at the details of your code coverage reports, such as the coverage breakdown by files, modules, classes, or functions; the coverage gaps or hotspots that indicate missing or insufficient tests; the coverage trends or changes over time or across versions; and the coverage distribution or variation among different test cases or scenarios. By thoroughly examining your code coverage reports, you can identify both the strengths and weaknesses of your unit tests, as well as potential opportunities and risks for improvement.

4 Use code coverage reports as a guide, not a goal

Code coverage reports can be a useful tool for guiding your unit testing efforts, however, they should not be your only measure of success. Code coverage reports tell you how much of your code is tested, but they cannot detect or prevent issues such as poor test design or implementation that leads to false positives or negatives. Additionally, code coverage reports cannot detect redundant, unreliable, or unmaintainable tests. To get the most out of code coverage reports, you should supplement them with other testing techniques and tools such as test review or inspection, test automation or continuous testing, test mutation or fuzzing, and test reporting or feedback. Doing so will help you achieve better unit testing outcomes and higher software quality.

5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?