Jest

https://jestjs.io/docs/
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### Repeating setup with beforeEach and afterEach

Source: https://jestjs.io/docs/setup-teardown

Use beforeEach and afterEach to run setup and teardown before and after each test. The example initializes and clears a city database for tests checking city names.

```javascript
beforeEach(() => {  
  initializeCityDatabase();  
});  
  
afterEach(() => {  
  clearCityDatabase();  
});  
  
test('city database has Vienna', () => {  
  expect(isCity('Vienna')).toBeTruthy();  
});  
  
test('city database has San Juan', () => {  
  expect(isCity('San Juan')).toBeTruthy();  
});  
```

--------------------------------

### One-time setup with beforeAll and afterAll

Source: https://jestjs.io/docs/setup-teardown

Use beforeAll and afterAll for setup that runs once per file. The example initializes and clears a city database that is reused across tests, with both hooks returning promises.

```javascript
beforeAll(() => {  
  return initializeCityDatabase();  
});  
  
afterAll(() => {  
  return clearCityDatabase();  
});  
  
test('city database has Vienna', () => {  
  expect(isCity('Vienna')).toBeTruthy();  
});  
  
test('city database has San Juan', () => {  
  expect(isCity('San Juan')).toBeTruthy();  
});  
```

--------------------------------

### Asynchronous beforeEach returning a promise

Source: https://jestjs.io/docs/setup-teardown

When setup is asynchronous, return a promise from beforeEach. The example returns initializeCityDatabase() which resolves when the database is initialized.

```javascript
beforeEach(() => {  
  return initializeCityDatabase();  
});  
```

--------------------------------

### Install @shelf/jest-dynamodb with Bun

Source: https://jestjs.io/docs/dynamodb

Install the @shelf/jest-dynamodb package using Bun as a dev dependency.

```bash
bun add --dev @shelf/jest-dynamodb  
```

--------------------------------

### setup-jest.js: configure jest-extended and afterEach hook

Source: https://jestjs.io/docs/configuration

Use this setup file with the setupFilesAfterEnv option to run code after the test framework is installed but before each test file. It adds matchers from jest-extended and resets to real timers after each test.

```javascript
const matchers = require('jest-extended');  

expect.extend(matchers);  

  

afterEach(() => {  
  jest.useRealTimers();  
});  

```

--------------------------------

### Install Jest with Bun

Source: https://jestjs.io/docs/getting-started

Install Jest as a dev dependency using Bun.

```bash
bun add --dev jest
```

--------------------------------

### Install @shelf/jest-dynamodb with pnpm

Source: https://jestjs.io/docs/dynamodb

Install the @shelf/jest-dynamodb package using pnpm as a dev dependency.

```bash
pnpm add --save-dev @shelf/jest-dynamodb  
```

--------------------------------

### Install Jest with pnpm

Source: https://jestjs.io/docs/getting-started

Install Jest as a dev dependency using pnpm.

```bash
pnpm add --save-dev jest
```

--------------------------------

### Install @babel/preset-typescript

Source: https://jestjs.io/docs/getting-started

Install @babel/preset-typescript with your preferred package manager to enable TypeScript support via Babel in Jest. Requires following the Babel setup instructions first.

```bash
npm install --save-dev @babel/preset-typescript
```

```bash
yarn add --dev @babel/preset-typescript
```

```bash
pnpm add --save-dev @babel/preset-typescript
```

```bash
bun add --dev @babel/preset-typescript
```

--------------------------------

### Install Jest with npm

Source: https://jestjs.io/docs/getting-started

Install Jest as a dev dependency using npm.

```bash
npm install --save-dev jest
```

--------------------------------

### Install @shelf/jest-mongodb with Bun

Source: https://jestjs.io/docs/mongodb

Install the @shelf/jest-mongodb preset as a dev dependency using Bun. This is an alternative to npm, Yarn, and pnpm.

```bash
bun add --dev @shelf/jest-mongodb  
```

--------------------------------

### Install @testing-library/react-native and test-renderer

Source: https://jestjs.io/docs/tutorial-react-native

Install the testing library and its peer dependency. `@testing-library/react-native` replaces the deprecated `react-test-renderer` and requires `test-renderer` to be installed alongside it.

```bash
yarn add --dev @testing-library/react-native test-renderer  
```

--------------------------------

### Install @shelf/jest-dynamodb with npm

Source: https://jestjs.io/docs/dynamodb

Install the @shelf/jest-dynamodb package using npm as a dev dependency.

```bash
npm install --save-dev @shelf/jest-dynamodb  
```

--------------------------------

### Install Jest with Yarn

Source: https://jestjs.io/docs/getting-started

Install Jest as a dev dependency using Yarn.

```bash
yarn add --dev jest
```

--------------------------------

### Install @shelf/jest-mongodb with pnpm

Source: https://jestjs.io/docs/mongodb

Install the @shelf/jest-mongodb preset as a dev dependency using pnpm. This is an alternative to npm and Yarn.

```bash
pnpm add --save-dev @shelf/jest-mongodb  
```

--------------------------------

### Install @shelf/jest-dynamodb with Yarn

Source: https://jestjs.io/docs/dynamodb

Install the @shelf/jest-dynamodb package using Yarn as a dev dependency.

```bash
yarn add --dev @shelf/jest-dynamodb  
```

--------------------------------

### Install @shelf/jest-mongodb with npm

Source: https://jestjs.io/docs/mongodb

Install the @shelf/jest-mongodb preset as a dev dependency using npm. This is the first step to use Jest with MongoDB.

```bash
npm install --save-dev @shelf/jest-mongodb  
```

--------------------------------

### Install Babel dependencies with Bun

Source: https://jestjs.io/docs/getting-started

Install the required dependencies to use Babel with Jest using Bun: babel-jest, @babel/core, and @babel/preset-env.

```bash
bun add --dev babel-jest @babel/core @babel/preset-env
```

--------------------------------

### Install @shelf/jest-mongodb with Yarn

Source: https://jestjs.io/docs/mongodb

Install the @shelf/jest-mongodb preset as a dev dependency using Yarn. This is an alternative to npm.

```bash
yarn add --dev @shelf/jest-mongodb  
```

--------------------------------

### Global setup script for custom Puppeteer integration

Source: https://jestjs.io/docs/puppeteer

Global setup script that launches Puppeteer and stores the browser instance and its WebSocket endpoint for later use by test environments and teardown.

```javascript
const {mkdir, writeFile} = require('fs').promises;
const os = require('os');
const path = require('path');
const puppeteer = require('puppeteer');

const DIR = path.join(os.tmpdir(), 'jest_puppeteer_global_setup');

module.exports = async function () {
  const browser = await puppeteer.launch();
  // store the browser instance so we can teardown it later
  // this global is only available in the teardown but not in TestEnvironments
  globalThis.__BROWSER_GLOBAL__ = browser;

  // use the file system to expose the wsEndpoint for TestEnvironments
  await mkdir(DIR, {recursive: true});
  await writeFile(path.join(DIR, 'wsEndpoint'), browser.wsEndpoint());
};
```

--------------------------------

### Install @babel/preset-env with Bun

Source: https://jestjs.io/docs/webpack

Install @babel/preset-env as a dev dependency using Bun. This is required for using Jest with webpack.

```bash
bun add --dev @babel/preset-env
```

--------------------------------

### Install Babel dependencies with pnpm

Source: https://jestjs.io/docs/getting-started

Install the required dependencies to use Babel with Jest using pnpm: babel-jest, @babel/core, and @babel/preset-env.

```bash
pnpm add --save-dev babel-jest @babel/core @babel/preset-env
```

--------------------------------

### Install Babel dependencies with npm

Source: https://jestjs.io/docs/getting-started

Install the required dependencies to use Babel with Jest: babel-jest, @babel/core, and @babel/preset-env.

```bash
npm install --save-dev babel-jest @babel/core @babel/preset-env
```

--------------------------------

### Install ts-jest

Source: https://jestjs.io/docs/getting-started

Install ts-jest, a TypeScript preprocessor with source map support for Jest. You may also need to create a configuration file for Jest to transpile TypeScript with ts-jest.

```bash
npm install --save-dev ts-jest
```

```bash
yarn add --dev ts-jest
```

```bash
pnpm add --save-dev ts-jest
```

```bash
bun add --dev ts-jest
```

--------------------------------

### Install jest-puppeteer with npm, Yarn, pnpm, or Bun

Source: https://jestjs.io/docs/puppeteer

Install the jest-puppeteer package using the package manager of your choice. These commands are alternatives; choose one based on your project's package manager.

```bash
npm install --save-dev jest-puppeteer
```

```bash
yarn add --dev jest-puppeteer
```

```bash
pnpm add --save-dev jest-puppeteer
```

```bash
bun add --dev jest-puppeteer
```

--------------------------------

### describe example: grouping tests for myBeverage

Source: https://jestjs.io/docs/api

Shows how to use describe to group related tests. The example defines a myBeverage object and tests that it is delicious and not sour. This is optional but helps organize tests.

```javascript
const myBeverage = {  

  delicious: true,  

  sour: false,  

};  

  

describe('my beverage', () => {  

  test('is delicious', () => {  

    expect(myBeverage.delicious).toBeTruthy();  

  });  

  

  test('is not sour', () => {  

    expect(myBeverage.sour).toBeFalsy();  

  });  

});  
```

--------------------------------

### webpack.config.js example

Source: https://jestjs.io/docs/webpack

A typical webpack configuration that uses babel-loader for JS/JSX, style-loader and css-loader for CSS, and asset modules for images and fonts. This is the starting point for translating a webpack setup to Jest.

```javascript
module.exports = {  
  module: {  
    rules: [  
      {  
        test: /\.jsx?$/,  
        exclude: ['node_modules'],  
        use: ['babel-loader'],  
      },  
      {  
        test: /\.css$/,  
        use: ['style-loader', 'css-loader'],  
      },  
      {  
        test: /\.gif$/,  
        type: 'asset/inline',  
      },  
      {  
        test: /\.(ttf|eot|svg)$/,  
        type: 'asset/resource',  
      },  
    ],  
  },  
  resolve: {  
    alias: {  
      config$: './configs/app-config.js',  
      react: './vendor/react-master',  
    },  
    extensions: ['.js', '.jsx'],  
    modules: [  
      'node_modules',  
      'bower_components',  
      'shared',  
      '/shared/vendor/modules',  
    ],  
  },  
};  
```

--------------------------------

### Install @jest/globals

Source: https://jestjs.io/docs/getting-started

Install @jest/globals to get type definitions for Jest global APIs that update with each Jest version. Then import the APIs from it in your test files.

```bash
npm install --save-dev @jest/globals
```

```bash
yarn add --dev @jest/globals
```

```bash
pnpm add --save-dev @jest/globals
```

```bash
bun add --dev @jest/globals
```

--------------------------------

### beforeAll - Set up global database before tests

Source: https://jestjs.io/docs/api

Runs a function before any tests in the file run. Useful for setting up global state used by many tests. Jest waits for a returned promise to resolve, enabling asynchronous setup. The default timeout is 5 seconds. If inside a describe block, runs at the beginning of that block. Use beforeEach to run setup before every test.

```javascript
const globalDatabase = makeGlobalDatabase();  

beforeAll(() => {  
  // Clears the database and adds some testing data.  
  // Jest will wait for this promise to resolve before running tests.  
  return globalDatabase.clear().then(() => {  
    return globalDatabase.insert({testData: 'foo'});  
  });  
});  

// Since we only set up the database once in this example, it's important  
// that our tests don't modify it.  
test('can find things', () => {  
  return globalDatabase.find('thing', {}, results => {  
    expect(results.length).toBeGreaterThan(0);  
  });  
});  
```

--------------------------------

### Failing example: variable not prefixed with 'mock'

Source: https://jestjs.io/docs/es6-class-mocks

This example will throw an out-of-scope error because the variable 'fakePlaySoundFile' does not start with 'mock'. Jest's hoisting prevents access to out-of-scope variables.

```javascript
// Note: this will fail  

import SoundPlayer from './sound-player';  

const fakePlaySoundFile = jest.fn();  

jest.mock('./sound-player', () => {  

  return jest.fn().mockImplementation(() => {  

    return {playSoundFile: fakePlaySoundFile};  

  });  

});  

```

--------------------------------

### Enable matcher for all tests via setupFilesAfterEnv

Source: https://jestjs.io/docs/expect

Shows how to move expect.extend to a setupFilesAfterEnv script to enable the custom matcher globally without importing in each test file.

```javascript
import {expect} from '@jest/globals';  
// remember to export `toBeWithinRange` as well  
import {toBeWithinRange} from './toBeWithinRange';  

expect.extend({  
  toBeWithinRange,  
});  
```

--------------------------------

### Generate basic configuration file with Bun

Source: https://jestjs.io/docs/getting-started

Run this command to generate a basic Jest configuration file using Bun. Jest will ask a few questions and create a configuration file with a short description for each option.

```bash
bunx create-jest
```

--------------------------------

### Snapshot output file (__tests__/__snapshots__/Intro-test.js.snap)

Source: https://jestjs.io/docs/tutorial-react-native

This is the snapshot file generated when running the test. It captures the rendered output of the Intro component. The snapshot should be committed and compared on subsequent test runs.

```javascript
exports[`Intro renders correctly 1`] = `  
<View  
  style={  
    Object {  
      "alignItems": "center",  
      "backgroundColor": "#F5FCFF",  
      "flex": 1,  
      "justifyContent": "center",  
    }  
  }>  
  <Text  
    style={  
      Object {  
        "fontSize": 20,  
        "margin": 10,  
        "textAlign": "center",  
      }  
    }>  
    Welcome to React Native!  
  </Text>  
  <Text  
    style={  
      Object {  
        "color": "#333333",  
        "marginBottom": 5,  
        "textAlign": "center",  
      }  
    }>  
    This is a React Native snapshot test.  
  </Text>  
</View>  
`;  
```

--------------------------------

### Install @types/jest

Source: https://jestjs.io/docs/getting-started

Install @types/jest to get types for Jest globals without importing them. This is a third-party library maintained at DefinitelyTyped, so match versions closely; e.g., use 27.4.x for Jest 27.4.0.

```bash
npm install --save-dev @types/jest
```

```bash
yarn add --dev @types/jest
```

```bash
pnpm add --save-dev @types/jest
```

```bash
bun add --dev @types/jest
```

--------------------------------

### Write test for DynamoDB insert and get

Source: https://jestjs.io/docs/dynamodb

Example test that inserts an item into the 'files' table and verifies it can be retrieved.

```javascript
it('should insert item into table', async () => {
  await ddb
    .put({TableName: 'files', Item: {id: '1', hello: 'world'}})
    .promise();

  const {Item} = await ddb.get({TableName: 'files', Key: {id: '1'}}).promise();

  expect(Item).toEqual({
    id: '1',
    hello: 'world',
  });
});
```

--------------------------------

### Install node-notifier with npm, Yarn, pnpm, or Bun

Source: https://jestjs.io/docs/configuration

Install node-notifier to enable native OS notifications for test results when notify is true. On macOS, allow notifications from terminal-notifier in System Preferences. On Windows, the first use creates a start menu entry and may not display the notification; subsequent runs work.

```bash
npm install --save-dev node-notifier
```

```bash
yarn add --dev node-notifier
```

```bash
pnpm add --save-dev node-notifier
```

```bash
bun add --dev node-notifier
```

--------------------------------

### afterAll - Clean up global database after all tests

Source: https://jestjs.io/docs/api

Runs a function after all tests in the file complete. Useful for cleaning up global setup state. The default timeout is 5 seconds. If inside a describe block, runs at the end of that block. Use afterEach instead to run cleanup after every test.

```javascript
const globalDatabase = makeGlobalDatabase();  

function cleanUpDatabase(db) {  
  db.cleanUp();  
}  

afterAll(() => {  
  cleanUpDatabase(globalDatabase);  
});  

test('can find things', () => {  
  return globalDatabase.find('thing', {}, results => {  
    expect(results.length).toBeGreaterThan(0);  
  });  
});  

test('can insert a thing', () => {  
  return globalDatabase.insert('thing', makeThing(), response => {  
    expect(response.success).toBeTruthy();  
  });  
});  
```

--------------------------------

### Set and get custom state with expect.setState() in JavaScript

Source: https://jestjs.io/docs/expect

Shows how to set a custom property using expect.setState() and retrieve it with expect.getState() in a JavaScript test file. The example logs the value 'value'.

```javascript
expect.setState({key: 'value'});  

const {key} = expect.getState();  

console.log(key); // 'value'  



```

--------------------------------

### Install identity-obj-proxy with npm, Yarn, pnpm, and Bun

Source: https://jestjs.io/docs/webpack

Install commands for the identity-obj-proxy package using npm, Yarn, pnpm, and Bun. This proxy is used to mock CSS Modules so className lookups return the class name as-is.

```bash
npm install --save-dev identity-obj-proxy  
```

```bash
yarn add --dev identity-obj-proxy  
```

```bash
pnpm add --save-dev identity-obj-proxy  
```

```bash
bun add --dev identity-obj-proxy  
```

--------------------------------

### Using .toHaveBeenNthCalledWith to test nth call arguments

Source: https://jestjs.io/docs/expect

Tests what arguments a mock function was called with on the nth call. The example ensures that drinkEach calls the drink function with 'lemon' first and 'octopus' second. The nth argument must be a positive integer starting from 1.

```javascript
test('drinkEach drinks each drink', () => {  
  const drink = jest.fn();  
  drinkEach(drink, ['lemon', 'octopus']);  
  expect(drink).toHaveBeenNthCalledWith(1, 'lemon');  
  expect(drink).toHaveBeenNthCalledWith(2, 'octopus');  
});  
```

--------------------------------

### Generate basic configuration file with npm

Source: https://jestjs.io/docs/getting-started

Run this command to generate a basic Jest configuration file. Jest will ask a few questions and create a configuration file with a short description for each option.

```bash
npm init jest@latest
```

--------------------------------

### Test nth return value with .toHaveNthReturnedWith(nthCall, value)

Source: https://jestjs.io/docs/expect

Use .toHaveNthReturnedWith(nthCall, value) to assert the value returned on the nth call. If that call threw an error, the matcher fails. The example checks the first and second returned beverage names. Note: nthCall must be a positive integer starting from 1.

```javascript
test('drink returns expected nth calls', () => {  

  const beverage1 = {name: 'La Croix (Lemon)'};  

  const beverage2 = {name: 'La Croix (Orange)'};  

  const drink = jest.fn(beverage => beverage.name);  

  

  drink(beverage1);  

  drink(beverage2);  

  

  expect(drink).toHaveNthReturnedWith(1, 'La Croix (Lemon)');  

  expect(drink).toHaveNthReturnedWith(2, 'La Croix (Orange)');  

});  
```

--------------------------------

### Use jest.requireActual to mock a module partially

Source: https://jestjs.io/docs/jest-object

Shows how to use jest.requireActual to get the real module inside a jest.mock factory, preserving the original exports while overriding a specific function. The TypeScript version adds a generic type parameter for type safety. The example demonstrates that getRandom always returns 10 because it is mocked, while other exports remain original.

```JavaScript
jest.mock('../myModule', () => {  

  // Require the original module to not be mocked...  

  const originalModule = jest.requireActual('../myModule');  

  

  return {  

    __esModule: true, // Use it when dealing with esModules  

    ...originalModule,  

    getRandom: jest.fn(() => 10),  

  };  

});  

  

const getRandom = require('../myModule').getRandom;  

  

getRandom(); // Always returns 10  
```

```TypeScript
jest.mock('../myModule', () => {  

  // Require the original module to not be mocked...  

  const originalModule =  

    jest.requireActual<typeof import('../myModule')>('../myModule');  

  

  return {  

    __esModule: true, // Use it when dealing with esModules  

    ...originalModule,  

    getRandom: jest.fn(() => 10),  

  };  

});  

  

const getRandom = require('../myModule').getRandom;  

  

getRandom(); // Always returns 10  
```

--------------------------------

### Example module with various data types (example.js)

Source: https://jestjs.io/docs/jest-object

A module exporting a function, async function, class instance, object, array, and primitives. Used to demonstrate how createMockFromModule mocks different data types.

```javascript
module.exports = {  
  function: function square(a, b) {  
    return a * b;  
  },  
  asyncFunction: async function asyncSquare(a, b) {  
    const result = (await a) * b;  
    return result;  
  },  
  class: new (class Bar {  
    constructor() {  
      this.array = [1, 2, 3];  
    }  
    foo() {}  
  })(),  
  object: {  
    baz: 'foo',  
    bar: {  
      fiz: 1,  
      buzz: [1, 2, 3],  
    },  
  },  
  array: [1, 2, 3],  
  number: 123,  
  string: 'baz',  
  boolean: true,  
  symbol: Symbol.for('a.b.c'),  
};  
```

--------------------------------

### Descriptive snapshot names example

Source: https://jestjs.io/docs/snapshot-testing

This example shows snapshots with descriptive names that describe the expected output, such as 'should render null' and 'should render Alan Turing'. It is used to illustrate the recommended practice of using descriptive names for easier review.

```javascript
exports[`<UserName /> should render null`] = `null`;  

  

exports[`<UserName /> should render Alan Turing`] = `  

<div>  

  Alan Turing  

</div>  

`;  
```

--------------------------------

### Snapshot test for Intro component (__tests__/Intro-test.js)

Source: https://jestjs.io/docs/tutorial-react-native

Uses `@testing-library/react-native` to render the Intro component and capture its output as a snapshot. The test is asynchronous and uses `screen.toJSON()` to get the rendered output.

```javascript
import {render, screen} from '@testing-library/react-native';  
import React from 'react';  
import Intro from '../Intro';  
  
test('renders correctly', async () => {  
  await render(<Intro />);  
  expect(screen.toJSON()).toMatchSnapshot();  
});  
```

--------------------------------

### jest-validate: validate example

Source: https://jestjs.io/docs/jest-platform

Validates a user's configuration against an example configuration using the validate function from jest-validate. The example passes a configuration with a custom transform and logs the result object containing hasDeprecationWarnings and isValid booleans.

```javascript
const {validate} = require('jest-validate');  

const configByUser = {  
  transform: '<rootDir>/node_modules/my-custom-transform',  
};  

const result = validate(configByUser, {  
  comment: '  Documentation: http://custom-docs.com',  
  exampleConfig: {transform: '<rootDir>/node_modules/babel-jest'},  
});  

console.log(result);  
```

--------------------------------

### Watch menu output with custom plugin entry

Source: https://jestjs.io/docs/watch-plugins

Example watch mode menu output after adding a custom plugin with key 's' and prompt 'do something'. The plugin's entry appears as 'Press s to do something.' in the menu.

```text
Watch Usage  

 › Press p to filter by a filename regex pattern.  

 › Press t to filter by a test name regex pattern.  

 › Press q to quit watch mode.  

 › Press s to do something. // <-- This is our plugin  

 › Press Enter to trigger a test run.  



```

--------------------------------

### createUser.js - fetch user creation

Source: https://jestjs.io/docs/bypassing-module-mocks

Example function that uses fetch to create a user and return the user ID. This is the code under test.

```javascript
import fetch from 'node-fetch';  

  
export const createUser = async () => {  
  const response = await fetch('https://website.com/users', {method: 'POST'});  
  const userId = await response.text();  
  return userId;  
};  

```

--------------------------------

### toBeNaN example

Source: https://jestjs.io/docs/expect

Use toBeNaN when checking that a value is NaN. The example shows that NaN passes and 1 does not.

```javascript
test('passes when value is NaN', () => {  
  expect(NaN).toBeNaN();  
  expect(1).not.toBeNaN();  
});
```

--------------------------------

### Generate basic configuration file with pnpm

Source: https://jestjs.io/docs/getting-started

Run this command to generate a basic Jest configuration file using pnpm. Jest will ask a few questions and create a configuration file with a short description for each option.

```bash
pnpm create jest
```

--------------------------------

### Install @babel/preset-env with pnpm

Source: https://jestjs.io/docs/webpack

Install @babel/preset-env as a dev dependency using pnpm. This is required for using Jest with webpack.

```bash
pnpm add --save-dev @babel/preset-env
```

--------------------------------

### Custom globalSetup and globalTeardown modules

Source: https://jestjs.io/docs/configuration

Define a custom global setup module (`setup.js`) and a matching teardown module (`teardown.js`). The setup function runs once before all test files and receives `globalConfig` and `projectConfig`; it can set global variables (e.g., `globalThis.__MONGOD__`) that are only readable in `globalTeardown`. The teardown function runs once after all tests and can clean up resources. Note that Jest does not transform code in `node_modules`.

```javascript
module.exports = async function (globalConfig, projectConfig) {
  console.log(globalConfig.testPathPatterns);
  console.log(projectConfig.cache);

  // Set reference to mongod in order to close the server during teardown.
  globalThis.__MONGOD__ = mongod;
};
```

```javascript
module.exports = async function (globalConfig, projectConfig) {
  console.log(globalConfig.testPathPatterns);
  console.log(projectConfig.cache);

  await globalThis.__MONGOD__.stop();
};
```

--------------------------------

### Install @babel/preset-env with Yarn

Source: https://jestjs.io/docs/webpack

Install @babel/preset-env as a dev dependency using Yarn. This is required for using Jest with webpack.

```bash
yarn add --dev @babel/preset-env
```

--------------------------------

### collectCoverageFrom: Coverage summary output

Source: https://jestjs.io/docs/configuration

Example output shown when `collectCoverageFrom` glob patterns do not match any files, resulting in 'Unknown%' coverage and the error 'Coverage data for global was not found.'

```text
=============================== Coverage summary ===============================  

Statements   : Unknown% ( 0/0 )  

Branches     : Unknown% ( 0/0 )  

Functions    : Unknown% ( 0/0 )  

Lines        : Unknown% ( 0/0 )  

================================================================================  

Jest: Coverage data for global was not found.  


```

--------------------------------

### Install @babel/preset-env with npm

Source: https://jestjs.io/docs/webpack

Install @babel/preset-env as a dev dependency using npm. This is required for using Jest with webpack.

```bash
npm install --save-dev @babel/preset-env
```

--------------------------------

### toMatch with regexp example

Source: https://jestjs.io/docs/expect

Shows how toMatch checks a string against a regular expression. Use when you need to verify a substring or pattern.

```javascript
describe('an essay on the best flavor', () => {  
  test('mentions grapefruit', () => {  
    expect(essayOnTheBestFlavor()).toMatch(/grapefruit/);  
    expect(essayOnTheBestFlavor()).toMatch(new RegExp('grapefruit'));  
  });  
});  
```

--------------------------------

### toBeUndefined example

Source: https://jestjs.io/docs/expect

Use toBeUndefined to check that a variable is undefined. The example checks that bestDrinkForFlavor('octopus') returns undefined.

```javascript
test('the best drink for octopus flavor is undefined', () => {  
  expect(bestDrinkForFlavor('octopus')).toBeUndefined();  
});
```

--------------------------------

### getMockImplementation() example in TypeScript

Source: https://jestjs.io/docs/mock-function-api

TypeScript version of the getMockImplementation() example, using the @jest/globals import and a typed mock function.

```TypeScript
import {jest} from '@jest/globals';  

const mockFn = jest.fn<() => number>();  

mockFn.getMockImplementation(); // undefined  

mockFn.mockImplementation(() => 42);  

mockFn.getMockImplementation(); // () => 42  
```

--------------------------------

### npm: install jest-environment-jsdom

Source: https://jestjs.io/docs/tutorial-jquery

Install the jest-environment-jsdom package using npm as a dev dependency to enable the JSDOM test environment.

```bash
npm install --save-dev jest-environment-jsdom  
```
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