Jest Cheat Sheet

Updated 2026-04-29

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Jest is a JavaScript testing framework developed by Meta (formerly Facebook) that has become the industry standard for testing JavaScript applications, especially in the React ecosystem. It provides a zero-configuration setup with a built-in test runner, assertion library, mocking utilities, and code coverage tools—all in one package. Jest 30 (released June 2025) brought significant performance improvements—up to 37% faster runs and 77% lower memory usage—along with native .mts/.cts support, improved ESM handling, and new features like expect.arrayOf and jest.advanceTimersToNextFrame. The key mental model: Jest isolates each test file in its own VM context, so side effects, globals, and module caches never leak between files—but this isolation comes at a startup cost worth understanding when optimizing large suites.

What This Cheat Sheet Covers

This topic spans 24 focused tables and 184 indexed concepts, 119 flashcards. Below is a complete table-by-table outline of this topic, spanning foundational concepts through advanced details.

Table 1: Core Test StructureTable 2: Setup and Teardown HooksTable 3: Common MatchersTable 4: Asymmetric MatchersTable 5: Negation and ModifiersTable 6: Function and Mock MatchersTable 7: Error TestingTable 8: Snapshot TestingTable 9: Mocking FunctionsTable 10: Mock Data InspectionTable 11: Spying on MethodsTable 12: Mocking ModulesTable 13: Mock CleanupTable 14: Async TestingTable 15: Timer MocksTable 16: Code CoverageTable 17: Configuration OptionsTable 18: CLI OptionsTable 19: React Testing Library IntegrationTable 20: Snapshot Best PracticesTable 21: Global Test ConfigurationTable 22: Custom MatchersTable 23: Advanced Mocking PatternsTable 24: Common Gotchas

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Table 1: Core Test Structure

The building blocks of every Jest file: describe groups tests, test/it defines them, and expect makes assertions. The variants — .each for parameterized runs, .skip/.only for focusing, .concurrent for parallelism, and .failing for documenting known bugs — let you shape a suite without rewriting the core logic.

Function	Example	Description
test / it	`test('adds 1 + 2', () => { expect(1 + 2).toBe(3); })`	• Defines a single test case • `it` is a fully interchangeable alias.
describe	`describe('Math', () => { test('add', () => {...}); });`	• Groups related tests into a suite • can be nested for hierarchical organization.
expect	`expect(value).toBe(3)`	• Creates an expectation object • always paired with a matcher like `toBe` or `toEqual`.
test.each	`test.each([[1,2,3],[2,3,5]])('adds %i+%i=%i',(a,b,e)=>{expect(a+b).toBe(e)})`	• Parameterized testing—runs the same logic with different data • reduces duplication.
test.skip	`test.skip('not ready', () => {...})`	• Skips a test • temporarily disables without deleting. Also `xit` / `xtest`.
test.only	`test.only('run this', () => {...})`	• Runs only this test in the file • all others skipped. Alias `fit`. Great for focused debugging.
test.concurrent	`test.concurrent('parallel', async () => {...})`	• Runs the test concurrently with other `concurrent` tests • requires async function.
test.failing	`test.failing('known bug', () => { expect(bugFn()).toBe(1); })`	• Expects the test to fail • test passes if it fails, fails if it passes. Documents known bugs.
describe.each	`describe.each([[1],[2]])('suite %i',(n)=>{...})`	Creates multiple describe blocks from a data table.
describe.only	`describe.only('focus suite', () => {...})`	• Runs only tests within this block • all other describes skipped.
describe.skip	`describe.skip('wip suite', () => {...})`	• Skips all tests in this block • alias `xdescribe`.

Table 2: Setup and Teardown Hooks

These four hooks run shared setup and cleanup around your tests. The key distinction is scope: the Each variants fire around every individual test (perfect for resetting state), while the All variants run once for the whole block (ideal for expensive one-time work like starting a server).

Hook	Example	Description
beforeEach	`beforeEach(() => { db = initDB(); })`	• Runs before each test in the current scope • ideal for resetting state.

The rest of this section is part of the full cheat sheet

Table 3: Common Matchers

Matchers are the assertions you chain after expect(), and these are the everyday workhorses. The one trap worth internalizing early is toBe versus toEqual — toBe checks reference or primitive identity while toEqual walks an object or array recursively — mixing them up is the most common source of confusing test failures.

Matcher	Example	Description
toBe	`expect(2 + 2).toBe(4)`	• Strict equality via `Object.is()` • for primitives and same-reference objects.

The rest of this section is part of the full cheat sheet

Table 4: Asymmetric Matchers

Asymmetric matchers are placeholders you drop inside a larger assertion to match part of a value loosely — "an object containing at least these keys," "a string matching this regex," "any number." They're invaluable when you care about some fields but want to ignore dynamic ones like IDs or timestamps, and they shine inside toHaveBeenCalledWith.

Matcher	Example	Description
expect.objectContaining	`expect(obj).toEqual(expect.objectContaining({id: 1}))`	• Matches any object with at least the specified keys • ignores extra properties.

The rest of this section is part of the full cheat sheet

Table 1: Core Test Structure

Function	Example	Description
test / it	`test('adds 1 + 2', () => { expect(1 + 2).toBe(3); })`	• Defines a single test case • `it` is a fully interchangeable alias.
describe	`describe('Math', () => { test('add', () => {...}); });`	• Groups related tests into a suite • can be nested for hierarchical organization.
expect	`expect(value).toBe(3)`	• Creates an expectation object • always paired with a matcher like `toBe` or `toEqual`.
test.each	`test.each([[1,2,3],[2,3,5]])('adds %i+%i=%i',(a,b,e)=>{expect(a+b).toBe(e)})`	• Parameterized testing—runs the same logic with different data • reduces duplication.
test.skip	`test.skip('not ready', () => {...})`	• Skips a test • temporarily disables without deleting. Also `xit` / `xtest`.
test.only	`test.only('run this', () => {...})`	• Runs only this test in the file • all others skipped. Alias `fit`. Great for focused debugging.
test.concurrent	`test.concurrent('parallel', async () => {...})`	• Runs the test concurrently with other `concurrent` tests • requires async function.
test.failing	`test.failing('known bug', () => { expect(bugFn()).toBe(1); })`	• Expects the test to fail • test passes if it fails, fails if it passes. Documents known bugs.
describe.each	`describe.each([[1],[2]])('suite %i',(n)=>{...})`	Creates multiple describe blocks from a data table.
describe.only	`describe.only('focus suite', () => {...})`	• Runs only tests within this block • all other describes skipped.
describe.skip	`describe.skip('wip suite', () => {...})`	• Skips all tests in this block • alias `xdescribe`.

Table 2: Setup and Teardown Hooks

Hook	Example	Description
beforeEach	`beforeEach(() => { db = initDB(); })`	• Runs before each test in the current scope • ideal for resetting state.

Table 3: Common Matchers

Matcher	Example	Description
toBe	`expect(2 + 2).toBe(4)`	• Strict equality via `Object.is()` • for primitives and same-reference objects.

Table 4: Asymmetric Matchers

Matcher	Example	Description
expect.objectContaining	`expect(obj).toEqual(expect.objectContaining({id: 1}))`	• Matches any object with at least the specified keys • ignores extra properties.