When comparing Karma vs CasperJS, the Slant community recommends CasperJS for most people. In the question“What are the best JavaScript unit testing tools?” CasperJS is ranked 3rd while Karma is ranked 4th. The most important reason people chose CasperJS is:
There are times where you don't want to open up a browser for screencaps, that is where CasperJS comes to use, it can render the page using its own rendering engine and take and save a screenshot for you, all via the commandline
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Pros
Pro Easily extensible
Do you prefer other test frameworks such as Mocha, Jasmine, qUnit or any other framework? Well you're in luck as Karma can be easiliy be extended to wrap around ANY framework of your choice.
Pro Provides both Browser based testing as well as headless tests
Karma eases out the UI testing process as you can test your code on all your devices let it be smartphones, tablets or your very own desktop. If you don't want all of that, you always have the option of headless testing using a PhantomJS instance.
Pro Provides the option of running client/servers either separately or on the Development computer
These options are really helpful in cases, where you have the luxury of multiple machines (tablets, mobile phones desktops) around you.
Pro You can test your code in cross browser environments
Being able to test your code directly via your testing tool is a breeze! You don't need to download a fancy tool to see how your app looks in a number of different browsers, now that Karma would do the job for you.
Pro Has plugins for WebStorm and is supported by the Netbeans IDE
Thanks to Karma, you won't need to spawn up a new terminal just so that you can test your app, you can now code and test right from the IDE
Pro Allows screenshots (either the full page or parts of it) if performing UI testing
There are times where you don't want to open up a browser for screencaps, that is where CasperJS comes to use, it can render the page using its own rendering engine and take and save a screenshot for you, all via the commandline
Pro Easy to understand
Pro Written in JavaScript
Since all webdevs know JS, the start-up time of learning the framework will be reduced to zero, as your team can be productive from day one.
Pro Easily integrates with other applications
Due to the simplicity of the framework, not only other libraries can be built with it, but it can be integrated with any web application as well.
Pro Can run javascript code inside pages being tested
Can execute arbitrary javascript or load external JS into the page being tested. This feature is possible due to the presence of a rendering engine, and helps you see the effects of any client side scripting during your tests.
Cons
Con No Support for NodeJS testing
Currently Karma doesn't support testing of apps built on NodeJS. So if you have a node app, you don't want to use Karma, Mocha or Jasmine can do the job for you.
Con No plugin for Eclipse (yet)
Do most of your code using Eclipse, well, you're in bad luck. Karma doesn't have an eclipse plugin, though if you are a real die hard eclipse fan, you can see this little hack to be able to run Karma from inside Eclipse Link
(Time of writing: July 2014)
Con Not for unit testing but rather UI testing
These are two extremely different concepts. CasperJS should be removed from this list
Con Cannot guarantee 100% accurate Webkit-based browser screenshots
QtWebKit is the rendering engine used by CasperJS. Keep in mind this is NOT the same rendering engine as Chrome; hence, if you want to be 100% sure of the results, you must run a Webkit browser (such as Chrome) yourself.