Alejandro Urdiales

Problem: Some interfaces like dates, times, localStorage, the URL, and the file system are “globals” that may change at any time. This makes it difficult to set up complex edge cases and hampers testing. Solution: Code to an interface instead of an implementation. (Instead of coding against these APIs directly, wrap access in an interface). Result: Now we can code and test against a mocked implementation in non-prod environments. The mocked implementation can be deterministic and fast, and thus allow quickly testing our business logic against many scenarios. In prod, the wrapper implementation merely forwards to the real APIs. Example: The Dropbox team used this pattern to test many OS and file system edge cases quickly on CI.