This post is a bit different from the recent ones I’ve published. I’m going to share my point of view on practicing DRY in unit tests and why I think it is bad for you. Care to know why? Here we go -

What is DRY?

Assuming that not all of us know what DRY means here is a quick explanation:
“Don't Repeat Yourself (DRY) is a principle of software development aimed at reducing repetition of software patterns” (from here). We don’t like duplications since “Duplication can lead to maintenance nightmares, poor factoring, and logical contradictions.” (from here).
An example can be having a single service which is responsible for fetching data from the server instead of duplicating the code all over the code base.
The main benefit is clear - a single source of logic, where each modification for it applies to all which uses it.

Where does DRY apply in tests?

In tests we thrive to assert as much as needed in order to give us the future modification confidence we feel comfortable with. This means that there will be a lot of tests that differ in nuances in order to make sure we cover each of the edge cases well.
What the previous sentence means in code is that tests tend to have a lot of repetitive and duplicated code to them, this is where the DRY principle finds its way in.

Let me try and explain with examples from the React world -
We are testing a custom component and we’re using the React Testing Library (and jest-dom) in order to test the component’s rendering. It may look something like this:

describe('Confirmation component', () => {
   it('should render', () => {
       const {getByRole} = render(<Confirmation />);
       expect(getByRole('dialog')).toBeInTheDocument();
   });
});

Here I’m testing that once the Confirmation component is being rendered, the element with the “dialog” role is present on the document.
This is great but it is just a single test among the many cases this component has, and that means that for each test you will have the same repetitive render code, which sometimes can be complex with props for the component, and perhaps wrapping it in a context provider.
So what many choose to do is to create a “helper” render function which encapsulates the rendering and then each test can call it, before starting its assertions:

function renderConfirmationComponent() {
   return render(<Confirmation />);
}

describe('Confirmation component', () => {
   it('should render', () => {
       const {getByRole} = renderConfirmationComponent();
       expect(getByRole('dialog')).toBeInTheDocument();
   });
});

We gain the benefit of DRY, where if we want to change the rendering for all the tests, we do it in a single place.

Another example of DRY in tests is using loops in order to generate many different test cases. An example can be testing an “add” function which receives 2 arguments and returns the result for it.
Instead of duplicating the code many times for each case, you can loop over a “data-provider” (or "data-set") for the test and generate the test cases, something like this:

describe('Add function', () => {
   const dataProvider = [
       [1, 2, 3],
       [3, 21, 24],
       [1, 43, 44],
       [15, 542, 557],
       [5, 19, 24],
       [124, 22, 146],
   ];

   dataProvider.forEach((testCase) => {
       it(`should return a ${testCase[2]} result for adding ${testCase[0]} and ${testCase[1]}`, () => {
           const result = add(testCase[0], testCase[1]);
           expect(result).toEqual(testCase[2]);
       });
   });
});

And the test result looks like this:

Add function
    ✓ should return a 3 result for adding 1 and 2 (1 ms)
    ✓ should return a 24 result for adding 3 and 21 (1 ms)
    ✓ should return a 44 result for adding 1 and 43
    ✓ should return a 557 result for adding 15 and 542
    ✓ should return a 24 result for adding 5 and 19 (1 ms)
    ✓ should return a 146 result for adding 124 and 22

BTW Jest even encourages you to do that with its built-in API, like test.each and describe.each.

Here is (somewhat) the same example with that API:

test.each(dataProvider)('.add(%i, %i)', (a, b, expected) => {
    expect(add(a, b)).toBe(expected);
});

Looks great, right? I created 6 test cases in just a few lines of code. So why am I saying it’s bad for you?

Searching

The scenario is typically this - a test fails, you read the output on the terminal and go searching for that specific failing test case. What you have in your hand is the description of the test case, but what you don’t know is that this description is a concatenation of strings.
You won't be able to find “should return a 3 result for adding 1 and 2” in the code because it simply does not exist. It really depends on how complex your test’s data-provider is, but this can become a real time waster trying to figure out what to search for.

Readability

So you found you test and it looks like this:

dataProvider.forEach((testCase) => {
       it(`should return ${testCase[2]} result for adding ${testCase[0]} and ${testCase[1]}`, () => {
           const result = add(testCase[0], testCase[1]);
           expect(result).toEqual(testCase[2]);
       });
});

You gotta admit that this is not intuitive. Even with the sugar (is it really sweeter?) syntax Jest offers it takes you some time to wrap your head around all the flying variables and string concatenations to realize exactly what’s been tested.
When you do realize what’s going on, you need to isolate the case which failed by breaking the loop or modifying your data-provider, since you cannot isolate the failing test case to run alone.
One of the best “tools” I use to resolve failing tests is to isolate them completely and avoid the noise from the other tests, and here it is much harder to do.
Tests should be easy to read, easy to understand and easy to modify. It is certainly not the place to prove that a test can be written in a one-liner, or with (god forbid) a reducer.

State leakage

Running tests in loops increases the potential of tests leaking state from one another. You can sometimes find out that after you’ve isolated the test which fails, it suddenly passes with flying colors. This usually means that previous tests within that loop leaked a certain state which caused it to fail.
When you have each test as a standalone isolated unit, the potential of one test affecting the others dramatically reduces.

The cost of generic code

Let’s go back to our React rendering example and expand it a little. Say that our generic rendering function receives props in order to render the component differently for each test case, and it might also receive a state “store” with different attributes to wrap the component with.
If, for some reason, you need to change the way you want to render the component for a certain test case you will need to add another argument to the rendering generic function, and your generic function will start growing into this little monster which needs to support any permutation of your component rendering.
As with any generic code, there is a cost of maintaining it and keeping it compatible with the evolving conditions.

Wrapping up

I know.
There are cases where looping over a data-provider to create test cases, or creating “helper” functions is probably the best way for achieving a good code coverage with little overhead. However, I would like you to take a minute and understand the cost of going full DRY mode in your tests given all the reasons mentioned above.
There is a clear purpose for your tests and that is to prevent regressions and provide confidence when making future changes. Your tests should not become a burden to maintain or use.
I much prefer simple tests, where everything which is relevant to a test case can be found between its curly brackets, and I really don’t care if that code repeats itself. It reassures me that there is little chance that this test is affected somehow by any side effect I’m not aware of.

As always, if you have any thoughts or comments about what's written here, please share with the rest of us :)

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