Test Doubles — Classify, Design & Create

Based on Martin Fowler's "Mocks Aren't Stubs" and Gerard Meszaros's xUnit Patterns taxonomy. Language-agnostic — works with any programming language and testing framework.

Test doubles replace real collaborators in tests. There are five distinct types, and choosing the wrong one is the most common cause of brittle, hard-to-maintain test suites. This guide helps you pick the right type, build it idiomatically, and avoid common traps.

Step 1: Understand Context

Before recommending or writing anything, establish these facts: