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Jest Cheat Sheet

A Quick Overview to Jest

Jest is a delightful JavaScript testing framework maintained by Facebook. It is widely used for unit testing, integration testing, and even end-to-end testing in modern JavaScript and Node.js applications. Jest comes with a powerful set of features, including a built-in test runner, assertion library, mocking utilities, snapshot testing, and test coverage reporting.

This guide targets Jest v20 and gives a concise yet practical overview of how to use it effectively.

Key Concepts Explained

• describe(): Used to group related test cases into a test suite.
• test() / it(): Define a single test case. it() is just a BDD-style alias for test().
• expect(): Assertion library that checks whether a value meets expectations.
• beforeEach() / afterEach(): Runs code before or after every test in a suite.
• beforeAll() / afterAll(): Runs setup/teardown code once before or after all tests.
• .only / .skip: Focus or ignore specific tests/suites.
• Snapshot testing: Captures the rendered output and compares it across test runs.
• Mock functions: Simulate function behavior or monitor how functions are called.
• Timer mocks: Test time-based behavior like setTimeout() and setInterval().
• Async testing: Write tests that handle Promises or async/await.

🚀 Quick Start

npm install --save-dev jest babel-jest

Add this to your package.json:

"scripts": {
  "test": "jest"
}

Run your tests:

npm test -- --watch

📖 See: Getting Started

✍️ Writing Tests

describe('My work', () => {
  test('works', () => {
    expect(2).toEqual(2);
  });
});

• describe: Group related tests.
• test or it: Define individual test cases.
• expect: Make assertions.

🔄 it() is an alias for test().

🔧 Setup Hooks

Use these for setup/teardown routines:

beforeEach(() => { ... });
afterEach(() => { ... });
beforeAll(() => { ... });
afterAll(() => { ... });

🎯 Focusing or Skipping Tests

Focusing tests:

describe.only(...);
it.only(...); // or fit()

Skipping tests:

describe.skip(...);
it.skip(...); // or xit()

🏁 Optional CLI Flags

✅ Expectations (Matchers)

Basic

expect(value).not.toBe(value);
expect(value).toEqual(value);
expect(value).toBeTruthy();

Note: toEqual performs deep equality.

Snapshots

expect(value).toMatchSnapshot();
expect(value).toMatchInlineSnapshot();

Inline snapshots require Prettier.

Errors

expect(fn).toThrow(error);
expect(fn).toThrowErrorMatchingSnapshot();

Booleans

expect(value).toBeFalsy();
expect(value).toBeNull();
expect(value).toBeTruthy();
expect(value).toBeUndefined();
expect(value).toBeDefined();

Numbers

expect(value).toBeCloseTo(number, digits);
expect(value).toBeGreaterThan(number);
expect(value).toBeLessThanOrEqual(number);

Objects

expect(value).toBeInstanceOf(Class);
expect(value).toMatchObject(obj);
expect(value).toHaveProperty('key', value);

Arrays/Strings

expect(value).toContain(item);
expect(value).toHaveLength(number);
expect(value).toMatch(/pattern/);

Custom Matchers

expect.extend(customMatchers);
expect.any(Constructor);
expect.assertions(1);

⏱️ Async Tests

Promises

test('resolves correctly', () => {
  return somePromise().then(data => {
    expect(data).toEqual(...);
  });
});

Async/Await

test('awaits correctly', async () => {
  const result = await asyncFunc();
  expect(result).toBe(...);
});

📖 See: Jest Async Testing

📸 Snapshot Testing

it('renders correctly', () => {
  const output = something();
  expect(output).toMatchSnapshot();
});

For React components:

import renderer from 'react-test-renderer';

it('matches snapshot', () => {
  const tree = renderer.create(<Component />).toJSON();
  expect(tree).toMatchSnapshot();
});

⏲️ Timers

jest.useFakeTimers();

it('delays call', () => {
  jest.runOnlyPendingTimers();
  jest.runAllTimers();
});

📖 See: Timer Mocks

🧪 Mock Functions

Creating Mocks

const fn = jest.fn();
const squared = jest.fn((n) => n * n);

Assertions

expect(fn).toHaveBeenCalled();
expect(fn).toHaveBeenCalledTimes(2);
expect(fn).toHaveBeenCalledWith(arg1, arg2);

Flexible Matchers

expect(fn).toHaveBeenCalledWith(expect.any(String));
expect(fn).toHaveBeenCalledWith(expect.arrayContaining([1, 2]));

Instances

const MyClass = jest.fn();
const a = new MyClass();
const b = new MyClass();
MyClass.mock.instances; // [a, b]

Call Data

fn.mock.calls.length;
fn.mock.calls[0][0];

Return Values

jest.fn().mockReturnValue('hello');
jest.fn().mockReturnValueOnce('hi');

Mock Implementations

const fn = jest
  .fn()
  .mockImplementationOnce(() => 1)
  .mockImplementationOnce(() => 2);

This comprehensive guide helps you get started with Jest testing or use it like a pro. It pairs perfectly with the React Testing Library for a user-centric testing workflow.