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# Contributing: Tests
> Testing is an opinionated topic. Here is a rough overview of our testing
> philosiphy. See something you want to discuss or think should be changed? Open
> a PR and let's discuss.
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Why do we write tests? The biggest and most important reason is _CONFIDANCE_ .
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## Kinds of Tests
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Test's buy us confidence, but not all tests are created equal. Each kind of test
has a different cost to write and maintain. More costly tests
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| Test Type | Example | Speed | Cost |
| ----------- | ------------------------------------------------------------------------------------------ | ---------------- | ------------------------------------------------------------------------ |
| Static | `addNumbers(1, '2')` was called with a `string` , `int` was expected. | :rocket: Instant | :money_with_wings: |
| Unit | `addNumbers(1, 2)` returns expected result `3` | :airplane: Fast | :money_with_wings::money_with_wings: |
| Integration | When I Click "Sign In", the page navigates to the dashboard (with mocked network requests) | :running: Okay | :money_with_wings::money_with_wings::money_with_wings: |
| End-to-end | When I click "Sign In", the page navigates to the dashboard (with no mocks) | :turtle: Slow | :money_with_wings::money_with_wings::money_with_wings::money_with_wings: |
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### Static Code Analysis
Modern tooling gives us this "for free". It can catch invalid regular
expressions, unused variables, and guarantee we're calling methods/functions
with the expected paramater types.
Example Tooling:
- [ESLint][eslint-rules]
- [TypeScript][typescript-docs] or Flow
Static code analysis can't test business logic.
### Unit Tests
...
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#### When should we unit test?
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Follow the top level exports. Anything that is exposed as public API should have
unit tests. These are th
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#### When should we avoid unit tests?
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You're testing implementation details if:
- Your test does something that the consumer of your code would never do.
- IE. Using a private function
- A refactor can break your tests
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### Integration Tests
...
### End-to-End Tests
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These are the most expensive tests to write and maintain. Largely because, when
they fail, they have the largest number of potential points of failure. So why
do we write them? Because they also buy us the most confidance.
We should reserve end-to-end tests for mission critical features. A good example
is testing user authentication. If a user can't sign in to your application,
it's an emergency. Having a high degree of confidance that users can always
authenticate is very valuable.
#### When should we test?
Mission critical features and functionality. Unsure if we should have a test for
feature `X` or scenario `Y` ? Open an issue and let's discuss.
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## Further Reading
- [Assert(js) Conf 2018 Talks][assert-js-talks]
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- [Write tests. Not too many. Mostly integration.][kent-talk] - Kent C. Dodds
- [I see your point, but…][gleb-talk] - Gleb Bahmutov
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- [Statc vs Unit vs Integration vs E2E Testing][kent-blog] - Kent C. Dodds
(Blog)
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<!--
Links
-->
<!-- prettier - ignore - start -->
[eslint-rules]: https://eslint.org/docs/rules/
[typescript-docs]: https://www.typescriptlang.org/docs/home.html
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<!-- Talks -->
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[assert-js-talks]: https://www.youtube.com/playlist?list=PLZ66c9_z3umNSrKSb5cmpxdXZcIPNvKGw
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[kent-talk]: https://www.youtube.com/watch?list=PLV5CVI1eNcJgNqzNwcs4UKrlJdhfDjshf
[gleb-talk]: https://www.youtube.com/watch?v=5FnalKRjpZk
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[kent-blog]: https://kentcdodds.com/blog/unit-vs-integration-vs-e2e-tests
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<!-- Images -->
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[testing-trophy]: https://twitter.com/kentcdodds/status/960723172591992832?ref_src=twsrc%5Etfw%7Ctwcamp%5Etweetembed%7Ctwterm%5E960723172591992832& ref_url=https%3A%2F%2Fkentcdodds.com%2Fblog%2Fwrite-tests
[aaron-square]: https://twitter.com/Carofine247/status/966727489274961920
[gleb-pyramid]: https://twitter.com/Carofine247/status/966764532046684160/photo/3
[testing-pyramid]: https://dojo.ministryoftesting.com/dojo/lessons/the-mobile-test-pyramid
[testing-dorito]: https://twitter.com/denvercoder/status/960752578198843392
<!-- prettier - ignore - end -->
