Flexible karma and protractor configuration without duplication

Whether you need to split a test suite or introduce variation into how it is run, but you depend on
configuration files (karma, protractor) to run them. Here's a sample project to show how to do that without duplicating a single piece of that configuration. Below you'll find a walk-through

Why split a test suite?

Often I split test suites in a project. One common use case is to have one unit test suite (stable, ultra-fast) and another one for focused integration, they are still low-level tests and basically asserts both that I integrate well with external web services, message-queues, etc (inherently unstable, quite fast).

It’s a bad idea to mix both in one test suite for many reasons, one of which is that they fail for different reasons. The focused integration tests rarely fail because of bugs my team introduced.

But now we have duplication!

https://www.flickr.com/photos/popilop/331357312

Anyway now that we have to test suites we need two karma.conf.js files, most of which will be duplicated.
Look at the following example, a very short snippet from a typical karma conf. Only the second entry of *files* will differ between test suites
It' a good thing that this is not only static conifiguration, but real code. If we have code then we have to power to eliminate duplication.

config.set({
    ...
    files: [
        'src'
        'unit-test/**/*.spec.js'
    ],
    
    // lots of other properties 
    ...
})

A solution

It is dead-simple:
extract what differs to karma-unit.conf.js, and karma-integration.conf.js.
These are the entry-points and they just extract a function to build the specifics into the overwhelmingly common part.
That’s what we’ll find in buildKarmaConf.js
Here from karma-integration.conf.js

    ...
    let testFiles = 'integration-test/**/*.spec.js'

    let karmaConfObject = buildKarmaConf(testFiles)
    config.set(karmaConfObject);
    ...

In buildKarmaConf we have something like

function buildKarmaConf (testFiles) {
    var sourceFiles = ['src']
    var allSourcesAndSomeTestFiles = sourceFiles.concat(testFiles)

    return {
        files: allSourcesAndSomeTestFiles,
        
        // lots of other properties 
        ...
    }
}

Another solution would be to use a environment variable and an if-statement instead of two *.conf.js files, but as in most of our code we have better alternatives (requiring less cyclomatic complexity), like what we're doing here - composing objects.

What if my situation is more complicated?

Compose further using the full power of a programming language (OO, FP). It is unlikely that the limited framework is going to solve the problem very efficiently.

What about protractor?

The exact same thing works

Conclusion

Karma and Protractor both use plain javascript to describe the test suite. That means we can use the the full power of JS to eliminate duplication just like in any other part of the application.