JavScript unit tests are run in a browser against a test server which actually handles the testing process.
This server allows you to capture a browser to run tests in, loads the test targets to the captured browser, controls the testing process, and exchanges data between the browser and PyCharm, so you can view test results without leaving the IDE.
The test server does not necessarily have to be on your machine, it can be launched right from PyCharm through a test runner. Currently, PyCharm supports integration with two test runners: JSTestDriver and Karma.
In this part: