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Creating a React component with TDD

Matti Bar-Zeev on November 05, 2021

Join me in this post as I create a React component using Test Driven Development (TDD) approach. I am going to create a confirmation component, wh...
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Bogdan Zaharia •

One thing that might help is too replace the expect(foo).toBeInTheDocument() with text assertion on elements. For example:

expect(getByRole('heading')).toHaveTextContent('Confirmation')
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This way, when there is an error, a typo for example, the output is more focused:

Expected element to have text content:
  Confirmation
Received:
  Confimation
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Idan Entin •

Great TDD walk through 👏
Once you added the tests for the buttons functionally, aren't the ones that test their existance redundant?

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Matti Bar-Zeev • • Edited

Cheers :)

Indeed, this is a common concern when practicing TDD, regardless of whether you test a UI component or a service. I mean, when TTD-ing a new service method you first wanna make sure the method is defined and can be accessed, while following tests use it to make sure it works as expected, so dose it make the initial test redundant? I does not IMO. Same when you test different usages of the method - some tests are bound to overlap.
The thing about TTD is that you build your service method or component one baby-step at a time, so first you wanna create the button and then you wanna attach some functionality to it. From my experience it has a lot to do with self-discipline and not jumping ahead of oneself. It is amazing how much frustration you save for yourself practicing it this way.
It can happen that multiple tests overlap and test the same code several times and that's ok. I think that it also makes it much more readable as a "spec" for someone who wants to create a component just by satisfying the tests:

  1. Confirmation component should render
  2. Confirmation component should have a title saying "Confirmation"
  3. Confirmation component should have a dynamic confirmation question
  4. Confirmation component should have an "OK" button
  5. Confirmation component should have an "Cancel" button
  6. Confirmation component should be able to receive a handler for the "OK" button and execute it upon click
  7. Confirmation component should be able to receive a handler for the "Cancel" button and execute it upon click
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Idan Entin • • Edited

I'd argue that for service it's the same case. So when testing a service, do you first check if the method exist?
While I understand how TDD by small steps helps focusing while building the component, I also appreciate the refactor part of the TDD cycle, and use it to refactor the code as well as my tests, and for me, clicking a button, requires it to be in the DOM so the test that checks this is redundant and I will therefore remove it in the refactor stage

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Matti Bar-Zeev •

At the end of the day, we do what helps us the most.
I like to think of the test I'm writing as a solid ground I blindly rely on and therefor I usually don't refactor them. I refactor the code it tests, knowing they protect me from my mistakes.
In my eyes tests should as simple as possible, with no implicit side effects. I see the the title of a failed test and I know right away what happened, no assuming or guessing, and yes - I write a test to make sure a service method is there, for I am willing to pay that price ;)

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Idan Entin •

Well TDD brings refactoring freesom, so you're missing out ;)
Regarding blindly relying on them, you can still do that, since the tests are also tested - by the code

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Alexander •

I guess, it can help you in the situation when you created a button, but forgot to attach an event listener.

Consider some big refactoring of the component – it probably can happen

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Idan Entin •

In such case the functionality test would fail thanks to getByRole

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Alexander •

In general you’re right, but if you won’t remove the first test, then both will fail. For some people it will be enough to understand a reason just by looking to the names of failed tests.

In your case developer will have to look in the message of failed test.

That’s how I understand the idea of writing tests with such granularity

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Idan Entin •

How many tests do you expect to fail if you make the button not available?
I think 2 failing tests are more confusing than one, even if I have to read the message (the title still helps me understand what failed - clicking the button)

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Nikhil Sengar •

Would snapshot testing not be the best thing to test the structure of component like if have tile present or not ?

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Matti Bar-Zeev • • Edited

That's a great question actually, even more so because of the documentation for Jest snapshots claims that they aim to help in testing a rendered UI, but what I found out over the time using snapshot for testing UI components is that it is (1) very hard to maintain, (2) when something breaks you need to look carefully at a complex structure and figure out if you expected that or not, and (3) most importantly for this post subject - you really cannot conduct TTD with snapshots (you can, but it is super hard to do, with low ROI).
I use snapshots to check outputs of serializable objects like arrays or json, you know, cause then it is easier to read and understand where the test failed and why.

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Nikhil Sengar •

got it , thanks for a detailed reply