Testing Elixir with Test Doubles: Stubs, Mocks, and Fakes

Testing concurrent Elixir applications often requires isolating external services, databases, and third-party APIs. To write fast, reliable tests without hitting real external networks, you must master the art of test doubles. This text-based course guides you through the concepts and practical application of stubs, mocks, and fakes specifically tailored for the Elixir ecosystem. You will transition from writing slow, fragile tests that rely on external state to crafting isolated, lightning-fast test suites that run concurrently without interference. What you'll learn: - Understand the core differences between stubs, mocks, fakes, dummies, and spies - Configure and use the Mox library to define mock behaviors for Elixir modules - Design clean Elixir behaviors and explicit contracts to decouple your application logic - Implement lightweight fakes for local development and unit testing - Manage concurrent tests safely using Mox's multi-process ownership model - Apply modern testing patterns that align with Elixir's asynchronous nature We begin with essential testing terminology and foundational concepts of dependency inversion. Next, you will explore step-by-step written tutorials that demonstrate how to define behaviors, swap implementations for testing, and assert on calls using Mox. Each concept is reinforced with practical code examples and conceptual walkthroughs. This course is designed for beginner to intermediate Elixir developers who want to improve their testing practices. No advanced testing experience is required, though a basic familiarity with Elixir syntax and the Mix build tool is recommended. Start writing robust, isolated Elixir tests today.