| name | jest-patterns |
| description | Jest and Vitest testing patterns including describe/it blocks, expect matchers, mocking, and async test strategies for JavaScript and TypeScript.
|
| requires | {"bins":["node"]} |
Jest and Vitest testing patterns for JavaScript and TypeScript projects.
When to activate
Use this skill when the project has jest.config.*, vitest.config.*,
a package.json listing jest or vitest as a dependency, or test files
matching *.test.{js,ts,tsx} or *.spec.{js,ts,tsx}.
Test file structure
- Use
describe blocks to group related tests by feature or function.
- Use
it (or test) blocks for individual test cases -- each one should
describe a single behavior.
- Nest
describe blocks for sub-scenarios (e.g. describe("when input is empty", ...) inside describe("parseConfig", ...)).
- Use
beforeEach / afterEach for per-test setup and teardown.
- Use
beforeAll / afterAll sparingly -- only for expensive one-time
setup like database connections.
Matchers
toBe(value) -- strict equality (===).toEqual(value) -- deep equality for objects and arrays.toMatchObject(subset) -- partial object matching.toThrow(error?) -- verify that a function throws.toHaveBeenCalledWith(args) -- assert mock was called with specific args.toMatchSnapshot() -- snapshot testing for serializable output.toContain(item) -- array/string includes check.toHaveLength(n) -- array or string length check.
Mocking
jest.fn() / vi.fn() -- create a mock function with call tracking.jest.mock("module") / vi.mock("module") -- replace an entire module
with auto-mocked version.jest.spyOn(obj, "method") / vi.spyOn(obj, "method") -- spy on a
method while keeping original implementation unless overridden.mockResolvedValue(val) -- shorthand for mocking async functions that
return promises.mockImplementation(fn) -- provide a custom implementation.
Async testing
- Use
async/await in test functions for promise-based code.
.resolves / .rejects matchers:
await expect(fetchData()).resolves.toEqual(expected).- Always
await assertions on promises -- forgetting await causes silent
passes.
Running
npx jest -- run all tests.npx jest --watch -- re-run on file changes.npx jest --testPathPattern="pattern" -- filter tests by file path.npx vitest -- run vitest in watch mode.npx vitest run -- single run without watch.
MUST
- Group related tests in
describe blocks with clear names.
- Use meaningful test descriptions that read as specifications.
- Clean up mocks in
afterEach (call jest.restoreAllMocks() or
vi.restoreAllMocks()).
- Test error paths -- not just the happy path.
MUST NOT
- Do not make real API calls -- mock HTTP with
jest.mock or msw.
- Do not share mutable state between tests -- each test must be independent.
- Do not snapshot everything -- snapshots are best for serializable output
that is tedious to assert manually, not for every return value.
