javascript-testing-patterns

Jest and Vitest are test frameworks that run unit tests for JavaScript and TypeScript. Both support configuration files (jest.config.ts, vitest.config.ts) where you define test environments, coverage thresholds, and module resolution. Choose Vitest for faster feedback in modern projects or Jest for broader ecosystem compatibility.

Mock API calls by intercepting module dependencies before tests run, then assert component behavior when data loads or errors occur. Testing Library's render function combined with mocked fetch or axios calls lets you test user-facing behavior without hitting real endpoints.

Testing Library provides utilities to render components and query them by user-visible elements—text, labels, roles—rather than implementation details. Write assertions on what users see and interact with, using patterns like render, fireEvent, and waitFor for async operations.

Dependency injection lets you pass mock implementations into functions and classes during tests, isolating units from external services. This approach works across pure functions, classes, and async services, reducing coupling and making tests deterministic.

Spy on async functions to verify they were called correctly, mock module exports to return test data, and use fixtures to provide consistent test inputs. Jest and Vitest support jest.spyOn() and vi.mock() respectively for controlling external dependencies.

Coverage thresholds define minimum percentages for lines, functions, and branches that must be tested. Configure them in jest.config.ts or vitest.config.ts to enforce quality standards without requiring 100% coverage, balancing test maintainability with risk management.