How to gather code coverage with Istanbul and Selenium and pitfalls to avoid

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Post summary: Istanbul does not seem to be recoding code coverage correctly, it turned out that the tests do navigation by changing the URL, which resets the code coverage.

How to use Istanbul for code coverage of Cypress automated tests was explained in detail in Testing with Cypress – Custom logging of errors and JUnit results post.

Code coverage with Istanbul and Selenium

Recently I had to do it again, this time with Selenium. There are several approaches, which can be taken to measure code coverage with Selenium. Whichever approach is taken, the first step is to instrument the frontend. How to do it with React and create-react-app is described in Testing with Cypress – Code coverage with Istanbul post. Coverage is present in __coverage__ JS frontend variable.

Once the frontend is instrumented, it is important to collect the code coverage after the tests are run. This is where approaches differ. One option is to use istanbul-middleware. In this case, a Node.js backend has to be created and the tests should post the coverage results, taken from __coverage__ to the backend. I find this approach not convenient, so I took the easier one.

Once the test is finished, the code coverage data is collected and saved as a JSON file in a test results folder, then all the results are used to generate the report. I use C# and the code to do so is as simple as:

public void CollectCodeCoverage()
		var data = ((IJavaScriptExecutor)_webDriver)
			.ExecuteScript("return window.__coverage__");
		if (data != null)
			var jsonString = JsonConvert.SerializeObject(data);
			var fileName = $"{_testResultsFolder}/coverage_{DateTime.Now.Ticks}.json";
			File.WriteAllText(fileName, jsonString);

Generating code coverage report

The report is generated with the nyc cli tool. Once all the JSON files are copied into a folder with the name .nyc_output, the command to run the report is nyc report –reporter=html. Nyc can be installed as a global NPM package or can be added to the frontend project inside package.json.

The issues measuring the code coverage

The setup described above is clear and easy to achieve. Although when tests were run, they did not record coverage, which was supposed to be there. I have spent several days trying to figure out what the issue was. And finally, I was able to understand. In my tests, I use _webDriver.Navigate().GoToUrl(). This actually visits a new URL, basically invalidating all the coverage results gathered so far. Once the problem was identified, the solution was pretty simple – save the cove coverage every time before a new URL is about to get opened.


Istanbul is a very good tool to measure the code coverage for web automation tests. In the current post, I have described a pitfall, which should be avoided when using it.

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