In previous lessons, we've explored the fundamentals of Test Driven Development (TDD), the Red-Green-Refactor cycle, and the setup of a testing environment. Now, we shift our focus to a key aspect of TDD: managing dependencies. Managing dependencies ensures that each unit of your application can be tested in isolation, which is crucial in TDD for maintaining code reliability and robustness.

In this lesson, we will examine how to use interfaces for abstraction in TypeScript, allowing us to manage dependencies effectively. Using simple examples, we will demonstrate how to apply the Red-Green-Refactor cycle in this context. We'll use TypeScript and Jest, popular tools in testing, while also mentioning alternatives like Mocha and Chai for wider context. Let’s dive in.

Dependencies in software development refer to the components or systems that a piece of code relies on to function properly. In the context of testing, dependencies can complicate unit tests because they might introduce external factors that affect the test outcomes. To ensure tests are isolated and independent, we use abstractions.

An interface in TypeScript acts as a contract that defines the structure a class should adhere to. By programming against interfaces, you can easily swap out implementations, making code more modular and test-friendly.

For example, consider a logger that a component uses to record actions. By abstracting the logger as an interface, you decouple the component from a specific logging implementation. This abstraction allows you to replace the actual logger with a mock or fake when testing, thus focusing on testing the component, not its dependencies.

We'll create a simple logger interface called ILogger to demonstrate dependency management. This interface will define a method log, which our will use: