Testing from the outside in
Given that inside-out TDD has some challenges as well as strengths, what difference does outside-in TDD make? This section reviews the alternative approach of starting from outside the system.
Outside-in TDD begins with the external users of the system. They may be human users or machines, consuming some API offered by our software. This approach to TDD begins by simulating some external input, such as the submission of a web form.
The test will typically use some kind of test framework – such as Selenium or Cypress for web applications – that allows the test to call up a specific web view, and simulate typing text into fields, then clicking a submit button. We can then make this test pass in the normal way, only we will have written some code that directly deals with the input from a user this time. In our hexagonal architecture model, we will end up writing the user input adapter first.
We can illustrate the outside-in approach as...
