Glossary

Jest · Reference cheat sheet

📋 Overview

Alphabetical glossary of core Jest terms for matchers, mocks, lifecycle hooks, snapshots, and test configuration.

🔧 Core concepts

Term	Definition
AfterAll	A hook that runs once after all tests in a file or `describe` finish.
AfterEach	A hook that runs after every test in the current scope.
BeforeAll	A hook that runs once before any tests in a file or `describe` start.
BeforeEach	A hook that runs before every test in the current scope.
Coverage	A report of which statements/branches tests executed.
Describe	A block that groups related tests under a shared name.
Each	A helper (`test.each` / `describe.each`) that table-drives repeated cases.
Expect	The assertion entry point that chains matchers onto a value.
Fake timers	Mocked clock APIs that let tests advance time manually.
Fn	`jest.fn()`, the factory for mock functions.
Hoisting	Jest’s behavior of lifting `jest.mock` calls to the top of a module.
It / test	A single test case function registered with Jest.
Matcher	An assertion method such as `toBe`, `toEqual`, or `toHaveBeenCalled`.
Mock	A stand-in function or module that records calls and controls returns.
Module mock	Replacing an imported module with a mock via `jest.mock`.
Only / skip	Focusing (`.only`) or excluding (`.skip`) specific suites or tests.
Setup file	A file run before the test framework is installed in each environment.
Snapshot	A serialized output stored on disk and compared on later runs.
Spy	A wrapper around a real function that records calls while optionally calling through.
Suite	A group of tests, typically created by a `describe` block.
Test environment	The runtime context such as `node` or `jsdom` for each test file.
Test runner	The component that discovers files and executes suites.
Timeout	The maximum duration a test may run before Jest fails it.
Transformer	A preprocessor (often Babel/ts-jest) that compiles files before tests.
Watch mode	A continuous mode that re-runs affected tests on file changes.

💡 Examples

Describe, test, and matchers:

describe("sum", () => {
  test("adds numbers", () => {
    expect(1 + 2).toBe(3);
    expect({ a: 1 }).toEqual({ a: 1 });
  });
});

Mocks and spies:

const fetchUser = jest.fn().mockResolvedValue({ id: 1 });
const log = jest.spyOn(console, "log").mockImplementation(() => {});

await fetchUser(1);
expect(fetchUser).toHaveBeenCalledWith(1);
log.mockRestore();

Lifecycle hooks:

beforeEach(() => {
  jest.clearAllMocks();
});

⚠️ Pitfalls

Confusing toBe (reference/primitive identity) with toEqual (deep value equality).
Mixing mock (full replacement) with spy (wrap existing implementation).
Treating snapshot failures as always product bugs — snapshots need intentional updates.
Equating fake timers with real delays — you must advance timers explicitly.
Assuming clearAllMocks resets implementations the same way resetAllMocks does.

Glossary

Glossary

📋 Overview

🔧 Core concepts

💡 Examples

⚠️ Pitfalls

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