Isolating Code

There are plenty of times when we can’t draw clear lines about what can be extracted into a new function. When that’s the case, we still have another tool at our disposal to create a controlled environment to exercise our code. We can create a replacement for the dependency, isolating our code.

Dependency Injection

Dependency injection (often abbreviated as DI) is a fancy name for any system that allows your code to utilize a dependency without hard-coding the dependency’s name, allowing any unit of code that meets a contract to be used.

In Elixir, we have two common ways to inject a dependency:

• As a parameter to a function
• Through the application environment

Utilizing DI in our tests allows us to create replacement dependencies that behave in predictable ways, focusing on the logic inside the code under test.