testing

How to reliably test async operations including promises, async/await, timers, and error rejections using Jest and Vitest patterns that prevent false positives and flaky tests.

Why testing matters, what unit tests are, what to test versus what to skip, and how automated tests improve code quality, enable safe refactoring, and serve as living documentation.

The essential Jest and Vitest matchers (toBe, toEqual, toMatch, toContain, toThrow) and how to choose the right assertion for each scenario to write clear, expressive test expectations.

Using beforeAll, beforeEach, afterEach, and afterAll hooks for test setup and teardown, understanding execution order, scope with describe blocks, and preventing test pollution.

How to organize test files, structure tests with describe and it blocks, write descriptive test names, apply the Arrange-Act-Assert pattern, and choose between co-located and centralized test directories.