Checks validate boolean conditions in your test. Testers often use checks to validate that the system is responding with the expected content. For example, a check could validate that a POST request has a response.status == 201, or that the body is of a certain size.
Checks are similar to what many testing frameworks call an assert, but failed checks do not cause the test to abort or finish with a failed status. Instead, k6 keeps track of the rate of failed checks as the test continues to run
Checks are great for codifying assertions relating to HTTP requests and responses. For example, this snippet makes sure the HTTP response code is a 200:
Sometimes, even an HTTP 200 response contains an error message. In these situations, consider adding a check to verify the response body, like this:
To verify the size of the response body, you can use a check like this:
When a script includes checks, the summary report shows how many of the tests' checks passed:
In this example, note that the check "is status 200" succeeded 100% of the times it was called.
You can also add multiple checks within a single check() statement:
When this test executes, the output will look something like this:
About Failing Checks
When a check fails, the script will continue executing successfully and will not return a 'failed' exit status. If you need the whole test to fail based on the results of a check, you have to combine checks with thresholds. This is particularly useful in specific contexts, such as integrating k6 into your CI pipelines or receiving alerts when scheduling your performance tests.