generating-test-doubles

name	generating-test-doubles
description	Generate mocks, stubs, spies, and fakes for dependency isolation. Use when creating mocks, stubs, or test isolation fixtures. Trigger with phrases like "generate mocks", "create test doubles", or "setup stubs".
allowed-tools	Read, Write, Edit, Grep, Glob, Bash(test:doubles-*)
version	1.22.0
author	Jeremy Longshore <[e-mail]>
license	MIT
tags	["testing","test-doubles"]
compatibility	Designed for Claude Code, also compatible with Codex and OpenClaw

Test Doubles Generator

Overview

Generate mocks, stubs, spies, and fakes to isolate units under test from external dependencies. Supports Jest mocks, Sinon.js stubs, Python unittest.mock, Go interfaces, and testdouble.js patterns.

Prerequisites

• Testing framework installed (Jest, Vitest, Mocha, pytest, JUnit 5, or Go testing)
• Mocking library available (Sinon.js, testdouble.js, unittest.mock, Mockito, or gomock)
• TypeScript strict mode enabled for type-safe mocks (if applicable)
• Source code with clear interface or class boundaries for dependency injection

Instructions

1. Scan the codebase with Glob and Grep to identify modules with external dependencies (database clients, HTTP clients, file system access, third-party SDKs).
2. Read each module under test and catalog its dependency interfaces -- list every method signature, return type, and side effect.
3. Determine the appropriate test double type for each dependency:
   • Stub: Returns canned data, no behavior verification (use for database queries returning fixed datasets).
   • Mock: Verifies interactions -- call count, argument matching, call order (use for email senders, event emitters).
   • Spy: Wraps real implementation while recording calls (use when partial behavior is needed).
   • Fake: Lightweight working implementation (use for in-memory repositories replacing real databases).
4. Generate test double files following the project's existing test directory structure (e.g., __mocks__/, test/doubles/, testutil/).
5. For each test double, implement:
   • Factory function or class matching the dependency interface exactly.
   • Configurable return values via builder pattern or method chaining.
   • Call recording for assertion (arguments, call count, call order).
   • Reset/restore mechanism for cleanup between tests.
6. Wire test doubles into existing test files using the framework's dependency injection pattern (jest.mock(), @patch, constructor injection, or Go interface substitution).
7. Validate all test doubles compile and pass type checks by running tsc --noEmit or equivalent.

Output

• Test double source files (one per dependency) placed in the project's mock directory
• Factory functions with TypeScript generics or equivalent type safety
• Jest auto-mock modules where applicable