Jest Mocking Patterns

About

This skill provides a comprehensive library of implementation patterns for Jest mocking, enabling developers to effectively isolate code under test from external side effects. It covers a wide spectrum of testing scenarios, from basic function mocks using jest.fn() and module-level overrides with jest.mock() to advanced techniques like spying on existing objects, simulating timers, and implementing dependency injection. By leveraging these patterns, developers can create faster, more reliable, and deterministic test suites that handle external APIs, complex asynchronous logic, and time-dependent code with ease.

Key Features

• Spying patterns for observing and overriding existing object methods
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• Advanced module mocking including partial mocks and factory functions
• Detailed mock assertions for verifying call counts and arguments
• Comprehensive coverage of jest.fn() and jest.mock() implementations
• Deterministic testing with fake timers and system date manipulation

Use Cases

• Implementing Dependency Injection patterns to improve code testability and modularity
• Testing complex asynchronous workflows, debouncing, and retry logic
• Isolating business logic from external API calls, databases, and third-party libraries