When debugging, a feature called LiveEdit allows you to change your code and have the changes immediately propagate into the browser where you are running your debug session. This saves time and helps avoid the common problem of trying to figure out why your change didn’t do anything, only to discover that you forgot to refresh your browser.
For unit testing, WebStorm bundles the JsTestDriver plug-in. This was originally a Google project, but JetBrains is now contributing to it. In addition, WebStorm can integrate with the Karma test runner. For either testing method, WebStorm tracks code coverage.
-- Martin Heller