When implementing a new feature, write your E2E tests first

A common misinterpretation of TDD and the Testing Pyramid is that unit tests are more important than E2E tests, and that you should start writing unit tests first. This is wrong. TDD only requires you to write tests first, but does not specify the type of test you must use. The Testing Pyramid simply encourages you balance your test suite to have more granular tests; it does not specify the importance or order for which you test.

In fact, when implementing a new feature, E2E tests are the most important tests, and should be the first test you write when composing your test suite. E2E tests mimic how your end users would interact with the project, and are often tied to the business requirements. If your E2E tests pass, it means the feature you are developing is working.

Moreover, it's often impractical to write your unit tests first. A unit tests concerns itself with implementation details, but there are many ways to implement a set of features and our first solutions are often substandard. It is likely to undergo many iterations of changes before it becomes stable. Since unit tests are coupled to the implementation they are testing, when the implementation changes, the unit tests would be discarded.

Therefore, when implementing new features, E2E tests should be written first; unit and integration tests should be written only after the implementation has settled.

Finally, E2E tests and unit tests are not mutually exclusive. For example, if you are writing a library that exports as single utility function, then your E2E tests are your unit tests.

This is because your end users would interact with your unit directly, making E2E and unit tests one and the same. Therefore, always keep your target audience in mind, and think about how they'll interact with your project. Use the appropriate type of tests to define contracts/interfaces with the end consumers, and use these test to drive your development.

Since we are developing new features, this chapter will focus on E2E tests. Unit and integration tests will be covered in the next chapter, TDD Part II: Unit/Integration Tests; and UI testing with Selenium will be covered in Chapter 15, E2E Testing with React. Manual tests are not programmable and thus earn only a brief mention at the end of this section.