Using Custom Jasmine Matchers to Make Unit Tests More Readable
I’m a stickler for the “single assertion per test” guideline. One of the pillars of good unit tests is readability. Multiple asserts undermine this principle and make tests that are more difficult to read, understand, and maintain. A clean solution to this problem is to use custom Jasmine matchers in place of multiple assertions.
Keep it DRY, Even (Especially?) in Tests
Consider the following:
In our app, we recieve a “split date” object from our service. It returns a value in the
This seems straight forward enough, but as it turns out we need to go the other direction as well:
This is bothersome. We have two stacks of
expect calls that are very similar, but different enough to require a bit more than a helper method. We definitely want to verify all of the properties of the results, but do we really need to do this individually? The short answer is ‘no.’
Enter the Custom Matcher
A custom matcher is nestled in a
beforeEach function. This will cause Jasmine to load the matcher prior to EVERY
it in your test suite.
this.addMatcher that takes an
object whose properties are your actual custom matchers. In our example the ‘toEqualDate’ matcher is the only property, but you can add as many as you might need for your application.
In the scope of the
toEqualDate function, we pass in the
expectedDate parameter. This is the argument passed into
toEqualDate(myExpectedDate). We also have access to
this.actual that is the argument passed into
expect(myActualDate) within your test.
The next important bit is the
this.message function that Jasmine will use to display any custom error messages. In this case we are returning two messages as appropriate. The first will return an invalid match, and the second will return a message if either object is not a valid date (if they are undefined, null, or incorrectly formatted).
toEqualDate function will return true or false based on the values we are comparing. If they are indeed valid
Date objects, it will compare them. If they are not, we return false.
At the top of the
beforeEach we also have two utility methods to clean up our custom matcher.
Keeping it Clean and DRY
I don’t know about you, but these tests look a lot better to me. While the overall lines of code may have increased, we’ve created a reusable solution that can be used over and over again while not repeating ourselves. Our tests now have a single
expect that is easy to understand and clearly expresses the intent of the test in an easy to read way.