AA-361: shift dates banner tests (#344)

docs/decisions/0007-testing.md

@@ -21,6 +21,31 @@ For example, they discourage using class or element name selectors to find compo
 during a test. Instead, you should find them by user-oriented attributes like labels,
 text, or roles. As a last resort, by a `data-testid` tag.
 
+## Mocking data
+We'll use [Rosie](https://github.com/rosiejs/rosie) as a tool for building JavaScript objects.
+Our main use case for Rosie is to use factories in order to mock the data we'd like to fetch when rendering components.
+[axios-mock-adapter](https://www.npmjs.com/package/axios-mock-adapter) allows us to mock the response of an HTTP request.
+
+For example, we may use a factory to build a course metadata object:
+
+`const courseMetadata = Factory.build('courseMetadata');`
+
+Then we'd pass that `courseMetadata` object into an axios mock call:
+
+`axiosMock.onGet('example.com').reply(200, courseMetadata);`
+
+This way, when a component sends a GET request to `example.com` within the test's lifecycle, the request will be intercepted
+by the axios-mock-adapter, and the courseMetadata object will be returned.
+
+These factories should live within the data directories they intend to mock
+```
+courseware
+  | data
+      | __factories__
+          | courseMetadata.factory.js /* used to define the Rosie factory */
+      | api.js /* getCourseMetadata() lives here */
+```
+
 ## What to Test
 We have not found exhaustive unit testing of frontend code to be worth the trouble.
 Rather, let's focus on testing non-obvious behavior.