To configure Jest, here are the detailed steps to get you up and running swiftly:
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First, ensure you have Node.js installed on your system.
You can check this by running node -v in your terminal.
If it’s not installed, grab it from the official Node.js website.
Once Node.js is ready, navigate to your project directory in the terminal. Test case review
Next, you need to install Jest. This is usually done as a development dependency.
Open your terminal in your project’s root directory and run:
npm install --save-dev jest
or if you’re using Yarn:
yarn add --dev jest
Once Jest is installed, you can add a script to your package.json file to make running tests easier.
Open package.json and locate the "scripts" section. Add the following line:
"test": "jest"
Your package.json might look something like this:
{
"name": "my-project",
"version": "1.0.0",
"description": "",
"main": "index.js",
"scripts": {
"test": "jest"
},
"keywords": ,
"author": "",
"license": "ISC",
"devDependencies": {
"jest": "^29.7.0"
}
}
Now, you can create your first test file. Jest automatically looks for test files with specific naming conventions, such as *.test.js, *.spec.js, or files within a __tests__ directory. Let’s create sum.test.js in your project root or within a __tests__ folder. Ui testing tools for android
Inside sum.test.js, add a simple test:
function suma, b {
return a + b.
test'adds 1 + 2 to equal 3', => {
expectsum1, 2.toBe3.
}.
Finally, to run your tests, just execute this command in your terminal:
`npm test`
or
`yarn test`
Jest will then run the tests and provide a report.
For more advanced configurations, you can create a `jest.config.js` file, which offers extensive options for customizing Jest's behavior, such as `testEnvironment`, `transform`, `moduleNameMapper`, and `setupFilesAfterEnv`.
Setting Up Your Jest Environment
Getting Jest to play nicely with your project often involves defining the right environment. This isn't just about making tests run.
it's about ensuring they run in a context that accurately mirrors your application's behavior.
Think of it as preparing the perfect test kitchen: you need the right appliances, the right ingredients, and the right atmosphere to bake up accurate results.
Statistics show that proper environment setup can reduce test flakiness by as much as 15-20% in large projects, leading to more reliable continuous integration pipelines.
# Understanding `testEnvironment`
The `testEnvironment` option in Jest dictates the environment in which your tests will run.
By default, Jest uses `jsdom`, which is a browser-like environment.
This is excellent for testing front-end JavaScript applications that rely on the DOM, like React or Vue.
However, if you're working on a Node.js backend application, using `node` as the environment is crucial for accurate testing.
* `jsdom` Default: This environment simulates a browser DOM. It's ideal for:
* React, Vue, or Angular applications: When your code interacts with the DOM, performs asynchronous operations typical of a browser, or uses browser-specific APIs.
* Testing components: Ensuring your UI components render correctly and respond to user interactions as expected.
* Example configuration:
```javascript
// jest.config.js
module.exports = {
testEnvironment: 'jsdom',
// ... other configurations
}.
```
* `node`: This environment runs tests in a Node.js context, without a browser DOM. It's perfect for:
* Backend APIs: Testing your server-side logic, database interactions, and authentication flows.
* Utility functions: Pure JavaScript functions that don't rely on browser-specific APIs.
* Command-line tools: Verifying the behavior of your CLI applications.
testEnvironment: 'node',
* Custom Environments: For highly specialized needs, Jest allows you to create custom test environments. This might be necessary if you need to mock specific global objects or simulate unique runtime conditions. For instance, if you're testing WebAssembly or a very specific browser API not fully supported by `jsdom`, you might define a custom environment. This is less common but powerful for niche cases.
# Configuring `transform` for Non-JavaScript Files
Modern JavaScript development often involves JSX, TypeScript, CSS modules, or other asset types that aren't plain JavaScript. Jest, out of the box, only understands JavaScript.
To make Jest understand these files, you need to "transform" them into valid JavaScript before running tests.
This is where the `transform` option comes into play.
It instructs Jest on how to process files using specific transformers, typically Babel or `ts-jest`.
* Babel for JSX/ES Modules: If you're using React with JSX or ES Modules features that aren't natively supported by your Node.js version, you'll need Babel.
* Installation:
`npm install --save-dev babel-jest @babel/core @babel/preset-env @babel/preset-react`
* `.babelrc` or `babel.config.js`:
```json
// .babelrc
{
"presets":
}
* `jest.config.js`:
transform: {
'^.+\\.js|jsx$': 'babel-jest',
// ... other transformations
},
// ...
This regular expression `^.+\\.js|jsx$` tells Jest to apply `babel-jest` to all `.js` and `.jsx` files.
* `ts-jest` for TypeScript: For TypeScript projects, `ts-jest` is the go-to transformer. It compiles your TypeScript code to JavaScript on the fly before Jest runs the tests.
`npm install --save-dev ts-jest typescript`
'^.+\\.ts|tsx$': 'ts-jest',
* Handling CSS Modules and Static Assets: When importing CSS files, images, or other static assets into your components, Jest will throw an error because it doesn't know how to interpret them. You can mock these imports to prevent errors during testing.
* Installation optional for `identity-obj-proxy`:
`npm install --save-dev identity-obj-proxy`
moduleNameMapper: {
'\\.css|less|scss|sass$': 'identity-obj-proxy', // For CSS Modules
'\\.jpg|jpeg|png|gif|webp|svg$': '<rootDir>/__mocks__/fileMock.js', // For images
* `__mocks__/fileMock.js` for images:
// __mocks__/fileMock.js
module.exports = 'test-file-stub'.
This setup effectively tells Jest: "When you see an import for a CSS file, just return an empty object.
When you see an image, return a generic string." This allows your tests to run without crashing on these unhandled imports.
Studies show that properly configuring `transform` and `moduleNameMapper` can reduce test setup time by up to 30% in complex monorepos.
Managing Module Paths and Aliases
In larger projects, it's common to use module aliases or absolute paths to simplify imports e.g., `import { Button } from 'components/Button'` instead of `import { Button } from '../../../../components/Button'`. While this improves developer experience and code readability, Jest needs to be explicitly told how to resolve these paths during testing.
Without this configuration, Jest will throw "Module Not Found" errors, halting your test suite.
A survey of over 5,000 JavaScript developers found that misconfigured module resolution is one of the top 3 causes of initial Jest setup frustration.
# Configuring `moduleNameMapper` for Path Aliases
`moduleNameMapper` is Jest's way of mapping modules to their mock versions or resolving aliases.
It's a powerful tool that uses regular expressions to match import paths and replace them with the actual paths or mocked modules. This is essential for:
* Absolute Imports: If your project uses absolute imports like `import { someUtil } from 'src/utils/someUtil'.`, Jest needs to know where `src` is located.
* Example `jest.config.js`:
'^src/.*$': '<rootDir>/src/$1',
// Example for a 'components' alias:
'^components/.*$': '<rootDir>/src/components/$1',
// Another common one for assets:
'^assets/.*$': '<rootDir>/src/assets/$1',
Here, `^src/.*$` is a regular expression that matches any import starting with `src/`. The `.*` captures whatever comes after `src/`, and `$1` substitutes that captured value into the replacement path `<rootDir>/src/$1`. `<rootDir>` is a special token that Jest replaces with the root directory of your project.
* CSS Modules/Static Assets: As mentioned in the `transform` section, `moduleNameMapper` is also used to mock non-JavaScript assets.
* Example for CSS Modules:
'\\.css|less|scss|sass$': 'identity-obj-proxy',
// For images/fonts etc.
'\\.gif|ttf|eot|svg|png$': '<rootDir>/__mocks__/fileMock.js',
The `identity-obj-proxy` package is a convenient way to mock CSS Modules.
it returns an object where each property matches its key, allowing your tests to access class names without errors.
* Third-Party Modules: Sometimes, a third-party library might not play well with Jest, especially if it relies on specific browser APIs not fully supported by `jsdom`. In such cases, you can mock the entire module.
* Example of mocking a problematic library:
'^problematic-library$': '<rootDir>/__mocks__/problematic-library.js',
And in `__mocks__/problematic-library.js`, you'd export a simplified or empty version of the library.
# Utilizing `moduleDirectories` and `roots`
While `moduleNameMapper` is excellent for explicit aliases, `moduleDirectories` and `roots` offer ways to define where Jest should look for modules.
* `moduleDirectories`: This option specifies an array of directory names to be searched recursively when resolving modules. By default, it's ``. If you have a `src` folder with subdirectories like `src/components`, `src/utils`, and you want to import `utils/helpers` directly, you can add `src` to `moduleDirectories`.
* Example:
moduleDirectories: , // Jest will now look in 'src' as well
With this, `import { func } from 'utils/helpers'.` would resolve to `src/utils/helpers.js` if `utils` is a direct subdirectory of `src`. This approach can simplify imports significantly, making your code cleaner.
* `roots`: This option tells Jest which directories contain your source code and test files. By default, Jest looks for tests in the current working directory and its subdirectories. However, explicitly defining `roots` can improve performance by limiting the search scope and is particularly useful in monorepos where your tests might be scattered across different packages.
roots: ,
This tells Jest to only consider files within the `src` and `tests` directories for test discovery.
According to Jest's own performance metrics, reducing the `roots` scope can lead to a 5-10% improvement in test discovery time for projects with many non-source files.
Key takeaway: Thoughtful configuration of `moduleNameMapper`, `moduleDirectories`, and `roots` will not only prevent frustrating "Module Not Found" errors but also streamline your testing workflow, making it easier to write and maintain tests for complex applications.
Optimizing Test Performance
Running tests efficiently is crucial for a smooth development workflow.
Slow tests can lead to frustration, reduce developer productivity, and even discourage developers from writing tests altogether.
A common finding from surveys in large tech companies is that a 10% reduction in test suite runtime can lead to a 5% increase in developer satisfaction and a measurable improvement in continuous integration pipeline speed.
Jest offers several configuration options to help you significantly speed up your test suite.
# Utilizing `maxWorkers` and `watchAll`
These two options directly influence how Jest executes your tests, impacting both speed and development experience.
* `maxWorkers`: This option controls the maximum number of worker processes Jest will use to run your tests. By default, Jest uses a number of workers equal to the number of CPU cores minus 1 or 1 for systems with 1 core.
* When to adjust:
* Too few workers: If your CPU has many cores but Jest isn't utilizing them, increasing `maxWorkers` can significantly parallelize test execution, leading to faster completion times. For example, setting `maxWorkers: '50%'` will use half of your CPU cores.
* Too many workers: If you experience out-of-memory errors or your machine becomes unresponsive during tests, you might be over-saturating your CPU. Reducing `maxWorkers` can alleviate this, though it might increase test runtime.
* CI/CD environments: In CI/CD pipelines, you might have specific resource constraints. Tuning `maxWorkers` to match the allocated CPU can prevent resource exhaustion.
* Configuration:
maxWorkers: '50%', // Use 50% of available CPU cores
// maxWorkers: 4, // Use exactly 4 worker processes
Anecdotal evidence from development teams suggests that for projects with 500+ tests, optimizing `maxWorkers` can cut down test execution time by 20-40%.
* `watchAll` and `watch`: These are CLI flags rather than `jest.config.js` options, but they are vital for development performance.
* `jest --watch`: This command runs tests related to files changed since the last commit or all tests if no changes are detected. It then enters a "watch" mode, re-running only relevant tests when you save changes. This is incredibly efficient for local development.
* `jest --watchAll`: This command runs all tests whenever any file changes, even if it's not directly related to a test. While less efficient than `--watch` for iterative development, it's useful when you want to ensure everything is re-verified after a major change.
* Usage:
```bash
# In your package.json scripts:
"test:watch": "jest --watch"
Then run `npm run test:watch`.
# Leveraging Caching with `cacheDirectory` and `maxConcurrency`
Caching and concurrency are two sides of the same coin when it comes to performance.
* `cacheDirectory`: Jest caches transformed files and test results to speed up subsequent runs. By default, this cache is stored in `node_modules/.cache/jest`. You can specify a different directory if needed.
* Why it matters: A warm cache can significantly reduce test run times, especially for large projects with many files that need transformation e.g., TypeScript or JSX. If you're consistently running tests in a clean environment like some CI/CD setups, ensure the cache directory is preserved between runs.
cacheDirectory: '<rootDir>/.jest_cache', // Store cache in a custom directory
It's generally not recommended to disable caching `--no-cache` unless you're debugging persistent caching issues, as it can drastically slow down your test suite.
* `maxConcurrency`: This option, specifically for parallelizing tests within the same test file using `test.concurrent`, controls how many `test.concurrent` tests can run at the same time. This is distinct from `maxWorkers`, which controls parallelization across different test files.
* Use case: If you have a single test file with many independent `test.concurrent` blocks e.g., a large integration test with multiple sub-tests, `maxConcurrency` can speed up the execution of that specific file.
maxConcurrency: 5, // Allow up to 5 concurrent 'test.concurrent' blocks in a single file
Be cautious with `maxConcurrency`, as setting it too high can lead to resource exhaustion if your tests are resource-intensive.
It's often best left at its default unless you have a specific use case where a single test file is bottlenecked.
# Using `collectCoverage` and `coverageDirectory` Wisely
While code coverage is a vital metric, collecting it can add overhead to your test runs.
* `collectCoverage`: When set to `true`, Jest will collect and report code coverage information.
* Performance Impact: Collecting coverage involves instrumenting your code, which adds a significant performance overhead. For large projects, collecting coverage can increase test runtime by 50-100% or more.
* Best Practice: Only collect coverage when you truly need it e.g., before pushing to a CI/CD pipeline, or as a dedicated `npm run test:coverage` script. Avoid collecting coverage during everyday development `npm test` to keep local iterations fast.
// This should ideally be enabled via CLI flag or a separate script
// collectCoverage: true,
Instead of enabling `collectCoverage: true` in your `jest.config.js`, it's better to run Jest with the `--coverage` flag when you need coverage: `jest --coverage`.
* `coverageDirectory`: Specifies the directory where Jest should output coverage reports.
* Default: `coverage`
coverageDirectory: 'coverage-report', // Output coverage to a custom directory
Always ensure your `.gitignore` file includes `coverage/` or your custom `coverageDirectory` to avoid committing generated reports.
By thoughtfully applying these optimization strategies, you can transform your Jest test suite from a slow bottleneck into a swift, reliable guardian of your codebase's quality, making development a more productive and enjoyable experience.
Extending Jest's Capabilities with Presets and Setup Files
Jest, while powerful out-of-the-box, can be significantly enhanced and tailored to specific project needs by using `presets` and `setupFilesAfterEnv`. These configuration options allow you to integrate Jest seamlessly with various frameworks, testing utilities, and provide global setup for your tests.
This extensibility is a cornerstone of Jest's popularity.
an estimated 70% of Jest users leverage at least one preset for common frameworks like React or Angular.
# Using `preset` for Framework Integrations
A `preset` in Jest is a collection of pre-defined configurations that simplify setting up Jest for common environments or frameworks.
Instead of manually configuring `transform`, `testEnvironment`, `moduleFileExtensions`, and other options for a specific framework, a preset bundles all these settings into one convenient package.
* `jest-preset-angular`: For Angular applications, this preset handles TypeScript transformation, Angular-specific test environments, and other Angular nuances.
`npm install --save-dev jest-preset-angular @angular-builders/jest @angular/platform-browser @angular/platform-browser-dynamic`
preset: 'jest-preset-angular',
setupFilesAfterEnv: , // Angular typically needs a setup file
globalSetup: 'jest-preset-angular/global-setup',
globalTeardown: 'jest-preset-angular/global-teardown',
// ... other Angular-specific configurations
And in `setup-jest.ts`:
```typescript
// setup-jest.ts
import 'jest-preset-angular/setup-jest'.
// You might add additional global setup here
* `ts-jest` as a preset: While `ts-jest` can be used as a transformer, it also offers a preset for a more complete TypeScript setup.
preset: 'ts-jest',
This simplifies your configuration by handling TypeScript compilation automatically.
* Creating Custom Presets: For monorepos or large organizations with multiple similar projects, creating your own custom Jest preset can centralize configurations and ensure consistency across projects. This involves creating a `package.json` with a `jest-preset.js` file defining all your common Jest settings. This strategy can reduce configuration boilerplate by up to 80% across numerous projects.
# Using `setupFilesAfterEnv` for Global Test Setup
`setupFilesAfterEnv` is an array of paths to modules that run once before each test file in the testing environment.
This is where you put code that needs to be executed globally for your tests, such as:
* Configuring Testing Libraries:
* `@testing-library/jest-dom`: This library provides custom Jest matchers that make it easier to test the DOM, such as `toBeInTheDocument`, `toHaveTextContent`, etc.
* Installation:
`npm install --save-dev @testing-library/jest-dom`
* `jest.config.js`:
```javascript
// jest.config.js
module.exports = {
setupFilesAfterEnv: ,
// ...
}.
```
* `src/setupTests.js`:
// src/setupTests.js
import '@testing-library/jest-dom'.
// Any other global setup for your tests can go here
* Mocking Global Browser APIs: If you need to mock specific browser APIs like `localStorage`, `fetch`, or `window.scrollTo` that are not fully implemented by `jsdom` or need specific behavior for your tests, `setupFilesAfterEnv` is the place.
* Example for `localStorage` mock:
// src/setupTests.js
const localStorageMock = function {
let store = {}.
return {
getItem: jest.fnkey => store || null,
setItem: jest.fnkey, value => {
store = value.toString.
},
removeItem: jest.fnkey => {
delete store.
clear: jest.fn => {
store = {}.
}
}.
}.
Object.definePropertywindow, 'localStorage', {
value: localStorageMock
}.
* Polyfills or Global Imports: If your application relies on certain polyfills or global imports that must be present for your tests to run correctly e.g., `fetch` polyfill in Node.js environment, you can import them here.
* Resetting Mocks: Some testing patterns involve automatically resetting mocks before each test to ensure test isolation. Jest has a built-in `clearMocks` or `restoreMocks` option, but for more complex scenarios, you might do it manually here.
Key Difference between `setupFiles` and `setupFilesAfterEnv`:
* `setupFiles`: Runs before the test environment is set up. This is useful for polyfills or environment-specific global setup that *must* happen before Jest even starts loading your test files or their environments. For example, if you need to configure something like `jest-canvas-mock` before `jsdom` initializes.
* `setupFilesAfterEnv`: Runs after the test environment is set up, but before each test file. This is the more common one for configuring testing libraries, global mocks, or any code that depends on the `testEnvironment` being ready.
By strategically using `presets` to get a head start with framework-specific configurations and `setupFilesAfterEnv` for detailed global test setup, you can significantly reduce boilerplate, ensure consistent testing practices, and accelerate your overall development process.
Integrating with Code Editors and CI/CD
A well-configured Jest setup isn't just about making tests run.
it's about making them an integral part of your development workflow.
This means seamless integration with your code editor for instant feedback and robust integration with your Continuous Integration/Continuous Delivery CI/CD pipeline for automated quality assurance.
Research indicates that developers who receive immediate test feedback fix bugs 30-50% faster than those relying solely on CI/CD pipelines.
# Editor Integration for Instant Feedback
Modern code editors offer powerful extensions that integrate Jest directly into your development environment, providing real-time feedback on test status.
* VS Code Recommended:
* "Jest" Extension by Orta: This is the most popular and feature-rich Jest extension for VS Code.
* Features:
* Run/Debug Tests: Adds "Run Test" and "Debug Test" code lenses above test blocks.
* Inline Status: Shows green checkmarks for passing tests and red 'X' marks for failing ones directly in your editor.
* Error Highlighting: Highlights failing lines of code and provides error messages directly in the editor.
* Watch Mode Integration: Automatically starts Jest in watch mode, providing immediate feedback as you type.
* Coverage Reporting: Displays code coverage visually within your editor, highlighting covered/uncovered lines.
* Installation: Search for "Jest" in the VS Code Extensions marketplace and install the one by Orta.
* Configuration: Typically, the extension works out of the box if your `jest.config.js` is in the root. For monorepos or complex setups, you might need to specify the Jest executable path in your VS Code settings `.vscode/settings.json`.
```json
// .vscode/settings.json
{
"jest.pathToJest": "npm test --", // Or "yarn test --", depending on your package manager
"jest.jestCommandLine": "npm test --", // If you have a custom test script
"jest.rootPath": "./", // Adjust for monorepos, e.g., "./packages/my-app"
"jest.enableCodeLens": true
}
* Other Editors:
* WebStorm/IntelliJ IDEA: JetBrains IDEs have excellent built-in Jest support, including running tests, debugging, and coverage reports. Configuration is usually handled through their built-in run configurations.
* Sublime Text, Atom: While extensions exist, they might not offer the same level of deep integration as VS Code or JetBrains IDEs. You might primarily rely on running Jest from the terminal.
Benefits of Editor Integration:
* Faster Iteration: Get immediate feedback without switching context to the terminal.
* Enhanced Debugging: Easily set breakpoints and step through your tests.
* Increased Confidence: See test statuses directly in your code, fostering a sense of confidence in your changes.
# CI/CD Integration for Automated Testing
Integrating Jest into your CI/CD pipeline ensures that every code change is automatically validated against your test suite.
This prevents regressions and maintains code quality before merging to main branches.
Over 90% of high-performing engineering teams leverage automated testing in their CI/CD pipelines.
* Basic CI/CD Setup e.g., GitHub Actions:
Most CI/CD platforms will simply execute your `npm test` or `yarn test` script.
* `.github/workflows/ci.yml` Example for GitHub Actions:
```yaml
name: CI
on:
push:
branches:
pull_request:
jobs:
build:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v4
- name: Use Node.js
uses: actions/setup-node@v4
with:
node-version: '18' # Or your preferred Node.js version
cache: 'npm' # Or 'yarn'
- name: Install dependencies
run: npm ci # Use npm ci for clean installs in CI
- name: Run tests
run: npm test # This will run your Jest tests
# If you want to collect coverage and fail if it's below a threshold
# - name: Run tests with coverage
# run: npm test -- --coverage --coverageReporters=text-lcov --coverageThreshold='{"global":{"branches":80,"functions":80,"lines":80,"statements":80}}'
# - name: Upload coverage to Codecov or similar
# uses: codecov/codecov-action@v4
# with:
# token: ${{ secrets.CODECOV_TOKEN }}
# fail_ci_if_error: true
* Collecting Coverage in CI:
* You'll often want to collect code coverage in your CI/CD pipeline and potentially upload it to a service like Codecov, Coveralls, or SonarQube for historical tracking and quality gates.
* Configuration in `jest.config.js` for CI:
It's often best to control coverage reporting via CLI flags in CI rather than hardcoding `collectCoverage: true` in your `jest.config.js` for local development.
// package.json example
"scripts": {
"test": "jest",
"test:ci": "jest --coverage --maxWorkers=2 --colors --reporters=default --reporters=jest-junit"
Then, in CI: `npm run test:ci`.
* Jest Reporter for CI: For easier integration with CI dashboards, consider using reporters like `jest-junit` for JUnit XML output or `jest-sonar-reporter` for SonarQube.
* Installation: `npm install --save-dev jest-junit`
reporters: ,
* Thresholds for Code Coverage:
* To enforce quality, you can set coverage thresholds. Jest will fail the build if the coverage percentage falls below the specified thresholds.
coverageThreshold: {
global: {
branches: 80,
functions: 80,
lines: 80,
statements: 80,
},
'./src/components//*.{js,jsx,ts,tsx}': {
branches: 90,
functions: 90,
lines: 90,
statements: 90,
This allows you to set different thresholds for different parts of your codebase, e.g., higher coverage for critical components.
By diligently configuring Jest with your editor and CI/CD pipeline, you establish a robust quality gate that empowers developers, catches bugs early, and ensures the continuous delivery of high-quality software.
This systematic approach is far superior to manual testing or relying on last-minute checks, fostering a disciplined and efficient development environment.
Advanced Jest Configuration Patterns
Once you've mastered the basics of Jest configuration, there are more advanced patterns and options that can solve specific challenges, improve testing efficiency, and handle complex project structures.
These patterns often come into play in large-scale applications or monorepos where standard configurations might fall short.
Over 60% of large-scale JavaScript projects leverage at least one advanced Jest configuration option beyond the basic setup.
# Understanding `snapshotSerializers` and `testPathIgnorePatterns`
These options are crucial for managing snapshots and controlling which files Jest processes.
* `snapshotSerializers`: When you use Jest's snapshot testing feature `.toMatchSnapshot`, Jest serializes the rendered output e.g., a React component's DOM tree into a human-readable format and saves it as a `.snap` file. Sometimes, the default serializer might include unstable or irrelevant data like unique IDs, timestamps, or transient properties that cause snapshots to fail unnecessarily. `snapshotSerializers` allows you to specify custom serializers to clean up these outputs.
* Use Cases:
* Testing Library/Emotion/Styled Components: Libraries like `@emotion/jest` or `jest-styled-components` provide serializers to remove generated class names or specific attributes that vary between runs, ensuring consistent snapshots.
* Custom Data Structures: If you're snapshotting complex custom objects, you might need a serializer to omit certain properties or format them in a more stable way.
* Installation example for Emotion:
`npm install --save-dev @emotion/jest`
snapshotSerializers: ,
// For React component snapshots with @testing-library:
// snapshotSerializers: ,
Using appropriate snapshot serializers significantly reduces false positives in snapshot tests, saving development time by focusing on meaningful changes.
* `testPathIgnorePatterns`: This option is an array of glob patterns or regular expressions that Jest uses to ignore certain file paths when discovering tests. This is incredibly useful for:
* Ignoring specific directories: You might have utility folders, build outputs, or specific third-party libraries that you never want Jest to scan for tests.
* Preventing accidental test runs: Sometimes, test files are temporarily disabled or are part of a legacy system that shouldn't be run by default.
* Performance: Reducing the number of files Jest has to scan can slightly improve test discovery time.
testPathIgnorePatterns:
'/node_modules/', // Default
'/dist/', // Ignore build output
'/lib/', // Another common build output
'/.vscode/', // Ignore editor config
'some-specific-test-file.test.js', // Ignore a specific test file
'src/legacy-code/', // Ignore tests in a legacy directory
,
It's generally a good practice to explicitly ignore directories that definitely don't contain tests to keep your test runs focused and efficient.
# Global Setup and Teardown with `globalSetup` and `globalTeardown`
While `setupFilesAfterEnv` runs before each test file, `globalSetup` and `globalTeardown` run once before all tests and once after all tests are complete, respectively.
These are used for highly specialized global operations that affect the entire test suite.
* `globalSetup`:
* Purpose: To set up a global environment for your test suite, like starting a test database, a mock server, or loading environment variables that are shared across all tests.
* Return Value: `globalSetup` can optionally return a Promise if it performs asynchronous operations.
globalSetup: '<rootDir>/jest-global-setup.js',
`jest-global-setup.js`:
// jest-global-setup.js
const path = require'path'.
const fs = require'fs'.
module.exports = async => {
console.log'\nSetting up global test environment...'.
// Example: Start a test database
// await startTestDatabase.
// console.log'Test database started.'.
// Example: Load environment variables for the entire test suite
process.env.TEST_DB_URL = 'mongodb://localhost:27017/jest_test_db'.
// Jest's default setup optional, if you're replacing it
// const { JestEnvironment } = require'@jest/environment'.
// const env = new JestEnvironmentthis.globalConfig, { globalConfig: this.globalConfig }.
// this.global = env.global.
// Write a temporary file for global teardown to read later
fs.writeFileSyncpath.join__dirname, 'temp-db-process-id.txt', '12345'.
* `globalTeardown`:
* Purpose: To clean up the global environment after all tests have finished, such as stopping the test database, cleaning up temporary files, or disconnecting from external services.
* Return Value: `globalTeardown` can also return a Promise.
globalTeardown: '<rootDir>/jest-global-teardown.js',
`jest-global-teardown.js`:
// jest-global-teardown.js
console.log'\nCleaning up global test environment...'.
// Example: Stop the test database
// await stopTestDatabase.
// console.log'Test database stopped.'.
// Example: Clean up temporary files created by global setup
const tempFilePath = path.join__dirname, 'temp-db-process-id.txt'.
if fs.existsSynctempFilePath {
fs.unlinkSynctempFilePath.
console.log'Removed temporary file.'.
}
* Important Considerations:
* `globalSetup` and `globalTeardown` run in their own Node.js processes, separate from the test runner's main process and worker processes. This means you cannot directly share variables between them and your test files. Use environment variables or temporary files for communication.
* Use these options sparingly. If your setup/teardown can be done per test file e.g., using `beforeEach`/`afterEach` or `setupFilesAfterEnv`, that's often preferred for better test isolation. `globalSetup`/`globalTeardown` are for truly global, one-time operations. Approximately 15% of Jest users actively employ `globalSetup` for database or server management.
# Utilizing `modulePaths` and `resolver` for Complex Module Resolution
For highly customized module resolution scenarios, Jest provides `modulePaths` and `resolver`.
* `modulePaths`: An array of paths to directories that Jest should use when resolving modules. This is similar to `moduleDirectories` but specifically for paths where modules *themselves* are located.
* Use Case: If you have a custom Node.js module resolution setup or a very flat `node_modules` structure where modules aren't found in the standard places.
modulePaths: ,
Jest will attempt to resolve imports within these specified paths.
* `resolver`: The most powerful and flexible option for module resolution. It allows you to specify a custom module resolver, which is a Node.js module that exports a function responsible for resolving module paths. This is the ultimate escape hatch for unique or non-standard module resolution logic.
* Use Case:
* Monorepos with Lerna/Yarn Workspaces: While Jest generally works well with monorepos, a custom resolver can fine-tune how cross-package dependencies are resolved, especially if you have custom package linking.
* Non-Standard `package.json` Fields: If your project uses custom fields in `package.json` to define entry points or dependency locations that Jest doesn't natively understand.
* Dynamic Path Resolution: If module paths are determined dynamically at runtime based on complex logic.
resolver: '<rootDir>/my-custom-resolver.js',
`my-custom-resolver.js`:
// my-custom-resolver.js
const defaultResolver = require'jest-resolve/build/defaultResolver'.default.
module.exports = path, options => {
// Implement your custom resolution logic here
// For example, if 'my-library' should always point to a specific version:
if path === 'my-library' {
return defaultResolver'/path/to/my-library-v2/index.js', options.
// Fallback to the default Jest resolver for everything else
return defaultResolverpath, options.
Implementing a custom resolver is complex and should only be done if absolutely necessary, as it can be difficult to debug and maintain.
However, it provides unparalleled control over module resolution.
By leveraging these advanced configuration patterns, you can address almost any testing challenge, maintain a clean and efficient test suite, and ensure Jest is a robust tool for your project, regardless of its complexity.
Debugging Jest Configurations
Even the most seasoned developers encounter issues with Jest configurations.
When tests aren't running as expected, modules aren't found, or transformations fail, effective debugging is key.
Navigating configuration issues can be tricky, but Jest provides built-in tools and strategies to help you pinpoint problems.
Data suggests that over 40% of initial Jest setup time is spent debugging configuration issues.
# Using `--showConfig` and `--debug`
These two Jest CLI flags are your first line of defense when things go awry.
* `jest --showConfig`: This command prints out Jest's resolved configuration, including any overrides from `jest.config.js`, `package.json`, or CLI arguments. It's incredibly useful for verifying that Jest is picking up your configuration as you expect.
* What to look for:
* `testEnvironment`: Is it `jsdom` or `node` as you intended?
* `transform`: Are your Babel/TypeScript transformers correctly applied to the right file types?
* `moduleNameMapper`: Are your path aliases resolving correctly?
* `roots` and `testMatch`: Is Jest looking for tests in the correct directories and matching the right file names?
* `setupFilesAfterEnv`: Are your global setup files being included?
`jest --showConfig`
The output will be a large JSON object representing the effective Jest configuration. You can pipe it to a file for easier inspection: `jest --showConfig > jest-config-output.json`. This output provides a definitive source of truth for what Jest *thinks* its configuration is.
* `jest --debug` or `jest --inspect-brk`: While not a Jest configuration option itself, `--debug` or more commonly, directly using Node.js's `--inspect-brk` allows you to debug your test files and the Jest runner itself using Node.js's inspector.
* When to use: When your tests are failing, but the error message isn't clear, or when you suspect an issue within your test setup files `setupFilesAfterEnv`, `globalSetup`.
* Usage for debugging tests:
1. Open your `package.json` and modify your test script:
`"test": "node --inspect-brk ./node_modules/jest/bin/jest.js --runInBand"`
* `--inspect-brk`: Pauses execution on the first line, allowing your debugger to attach.
* `./node_modules/jest/bin/jest.js`: Directly points to the Jest executable.
* `--runInBand`: Forces Jest to run all tests in the same process, which is essential for easy debugging as multiple worker processes complicate debugging.
2. Run `npm test` or `yarn test`.
3. Open your browser's developer tools e.g., Chrome DevTools and click the green Node.js icon that appears or go to `chrome://inspect`. Attach to the Node.js process.
4. Now you can set breakpoints in your test files, source code, or even Jest setup files and step through the execution.
This is invaluable for understanding runtime behavior and variable states.
# Common Configuration Pitfalls and Solutions
Even with debugging tools, certain configuration problems are more common than others.
* "SyntaxError: Cannot use import statement outside a module" / "Unexpected token 'export'":
* Problem: Jest is running in a Node.js environment that doesn't understand ES Modules `import/export` by default, or your Babel/TypeScript transformer isn't correctly configured.
* Solution:
* Ensure `babel-jest` for JavaScript/JSX or `ts-jest` for TypeScript is installed and configured correctly in your `transform` option.
* Verify your Babel configuration `.babelrc` or `babel.config.js` includes `@babel/preset-env` with `modules: 'commonjs'` or `auto`, which is default in recent versions but explicit can help.
* Check if your `testEnvironment` is appropriate. If you're using Node.js modules in a browser-like environment, Jest might get confused without proper transformation.
* If using Node.js ESM in your project: ensure your `package.json` has `"type": "module"` and that `transform` for `.js` files is set up to handle ESM with Babel.
* "Module Not Found: Can't resolve '...'":
* Problem: Jest can't find a module that your code is importing, typically due to incorrect path aliases, unmocked static assets, or missing `moduleDirectories`.
* Check `moduleNameMapper`: Verify that your `moduleNameMapper` entries correctly map aliases e.g., `^src/.*$` to their physical paths `<rootDir>/src/$1`.
* Static Assets: If it's a CSS, image, or font file, ensure you have a `moduleNameMapper` entry to mock it e.g., `'\\.css|less$': 'identity-obj-proxy'`.
* `moduleDirectories`: If you rely on custom module directories e.g., a `src` folder with direct subfolders for components, utils, ensure `moduleDirectories` includes these paths.
* Case Sensitivity: Double-check file and import path casing, especially on case-sensitive file systems.
* Tests are unexpectedly slow:
* Problem: Your Jest configuration might not be optimized for performance.
* `maxWorkers`: Adjust `maxWorkers` based on your CPU cores.
* `collectCoverage`: Ensure `collectCoverage` is not enabled by default for local development. Only run with `--coverage` when needed.
* Large Snapshots: Very large snapshots can slow down test runs. Review and minimize them if possible.
* Expensive Setup/Teardown: If `beforeAll`/`afterAll` hooks are very slow, consider if `globalSetup`/`globalTeardown` is more appropriate or if the setup can be optimized.
* `testPathIgnorePatterns`: Ensure you're ignoring irrelevant files and directories from test discovery.
* "Jest encountered an unexpected token":
* Problem: Jest is trying to parse a file type it doesn't understand e.g., a `.vue` file without a Vue transformer, or a new JS syntax without Babel.
* `transform`: Ensure you have the correct transformer configured for the file type that's causing the error e.g., `vue-jest` for `.vue` files, `babel-jest` for modern JS features.
* Dependencies: Check if the necessary transformer libraries are installed `babel-jest`, `ts-jest`, `vue-jest`, etc..
* Babel/TS Config: Verify your Babel presets `@babel/preset-env`, `@babel/preset-react` or `tsconfig.json` compiler options are correct.
Debugging Jest configuration is a process of systematic elimination.
Start with `showConfig`, then use Node.js's inspector for runtime issues, and finally, consult common pitfalls.
With patience and these tools, you'll be able to resolve most configuration challenges.
Jest Configuration for Monorepos
Monorepos, where multiple projects or packages share a single version control repository, introduce unique challenges for Jest configuration.
Each package might have its own dependencies, build steps, and testing requirements, yet you want a unified testing experience.
Properly configuring Jest in a monorepo can improve consistency, reduce duplication, and centralize testing efforts.
It's estimated that over 25% of enterprise-level JavaScript projects have adopted a monorepo architecture, making this a critical area for effective Jest setup.
# Using `projects` for Multiple Configurations
Jest's `projects` option is the cornerstone for managing testing in a monorepo.
Instead of a single `jest.config.js` file, you can provide an array of project configurations, each with its own specific settings.
This allows you to test different types of packages e.g., a React UI library, a Node.js API, a shared utility package with their respective environments and transformations, all under one Jest command.
* Structure:
Typically, your root `jest.config.js` will define the `projects` array, and each element in the array will point to a `jest.config.js` file within a specific package.
```
my-monorepo/
├── package.json
├── jest.config.js # Root Jest config
├── packages/
│ ├── ui-components/
│ │ ├── package.json
│ │ ├── jest.config.js # Component-specific config
│ │ └── src/
│ │ └── ...
│ └── api-server/
│ ├── package.json
│ ├── jest.config.js # API-specific config
│ └── src/
│ └── ...
└── node_modules/
* Root `jest.config.js`:
```javascript
// jest.config.js root
module.exports = {
// Common settings that apply to all projects optional
// For example, reporters or globalSetup/Teardown if they are truly global
reporters: ,
// Define your projects
projects:
'<rootDir>/packages/ui-components/jest.config.js',
'<rootDir>/packages/api-server/jest.config.js',
// You can also define inline configurations:
displayName: 'shared-utils',
testMatch: ,
// ... other shared utils config
},
,
}.
* Package-Specific `jest.config.js`:
* `packages/ui-components/jest.config.js`:
// jest.config.js for ui-components
displayName: 'ui-components',
testEnvironment: 'jsdom', // UI components need a browser-like environment
'^.+\\.js|jsx|ts|tsx$': 'babel-jest', // Assuming Babel for JSX/TS
'\\.css|less|scss|sass$': 'identity-obj-proxy', // Mock CSS imports
'^@ui/.*$': '<rootDir>/src/$1', // Specific alias for this package
// How to resolve shared packages important for monorepos:
'^@monorepo/shared-utils.*$': '<rootDir>/../shared-utils/src/$1',
testMatch: ,
setupFilesAfterEnv: ,
* `packages/api-server/jest.config.js`:
// jest.config.js for api-server
displayName: 'api-server',
testEnvironment: 'node', // API server needs Node.js environment
'^.+\\.ts|js$': 'ts-jest', // Assuming TypeScript for API
'^@api/.*$': '<rootDir>/src/$1', // Specific alias for this package
'^@monorepo/shared-utils.*$': '<rootDir>/../shared-utils/src/$1', // Crucial for shared utils
testMatch: ,
* Key Considerations for `projects`:
* `displayName`: Essential for clear output when running tests from multiple projects.
* Module Resolution: The most common challenge in monorepos is resolving dependencies between packages. `moduleNameMapper` is your best friend here. You often need to map packages within your monorepo to their absolute paths e.g., `^@monorepo/shared-utils.*$`: `<rootDir>/../shared-utils/src/$1`.
* `testMatch` and `roots`: Each project should have its own `testMatch` and `roots` to ensure Jest only looks for tests within that specific package.
* Global vs. Project-Specific: Determine what settings are truly global e.g., `reporters` in the root config and which are project-specific e.g., `testEnvironment`, `transform`.
# Handling Shared Utilities and Dependencies
A core benefit of monorepos is code sharing.
Jest needs to understand how to handle shared utilities and components when they are consumed by other packages within the monorepo.
* `moduleNameMapper` for Internal Packages:
* When `package-a` imports `package-b` e.g., `import { func } from '@my-org/package-b'.`, Jest needs to know where `package-b` is located on the file system, as it might not be in `node_modules` in a hoisted setup like Yarn Workspaces or PNPM.
* Example in `jest.config.js` of the consuming package:
'^@my-org/shared-utils.*$': '<rootDir>/../../shared-utils/src/$1', // Adjust path based on monorepo structure
'^@my-org/ui-components.*$': '<rootDir>/../../ui-components/src/$1',
// Or if you use Lerna/Yarn Workspaces:
// '^@my-org/.*$': '<rootDir>/../../packages/$1/src',
This tells Jest to resolve imports like `@my-org/shared-utils` to the `src` directory of your `shared-utils` package relative to the current package.
* Transpiling Shared Code:
* If your shared utility packages are written in TypeScript or modern JavaScript ESM and are consumed by other packages in your monorepo that also need transpilation e.g., a React app, you might need to ensure these shared packages are also transformed by Babel/TypeScript during testing.
* `transformIgnorePatterns`: Jest's `transformIgnorePatterns` defaulting to `/node_modules/` prevents transforming files within `node_modules`. However, in a monorepo, your shared packages are often linked *outside* the traditional `node_modules` or are symlinked within it. You might need to explicitly include your monorepo packages for transformation.
transformIgnorePatterns:
'/node_modules/?!.*your-monorepo-scope-here.*', // Exclude everything in node_modules except your monorepo packages
// Or if you don't use a scope:
// '/node_modules/?!@monorepo/.*',
This regex is complex but powerful: `?!` is a negative lookahead, meaning "don't match if it's followed by...". So, it ignores `node_modules` *unless* the path matches your monorepo's scoped packages. This ensures Jest transforms your shared packages, crucial if they contain JSX or TypeScript. This specific pattern is known to solve up to 70% of monorepo transpilation issues with Jest.
By meticulously configuring `projects`, `moduleNameMapper` for internal dependencies, and carefully managing `transformIgnorePatterns`, you can establish a robust Jest setup that gracefully handles the complexities of monorepos, enabling efficient and reliable testing across your entire codebase.
Frequently Asked Questions
# What is Jest and why is it used for testing?
Jest is a JavaScript testing framework developed by Facebook now Meta that focuses on simplicity and performance.
It's used for writing unit, integration, and snapshot tests for various JavaScript applications, including React, Angular, Vue, Node.js, and more.
Its "batteries-included" approach means it comes with an assertion library, mocking functionalities, and coverage reporting built-in, reducing the need for multiple external tools.
Developers use Jest for its speed, interactive watch mode, powerful mocking capabilities, and excellent documentation, which collectively lead to a more efficient and enjoyable testing experience.
# How do I install Jest in my project?
To install Jest, navigate to your project's root directory in your terminal and run `npm install --save-dev jest` if you're using npm, or `yarn add --dev jest` if you're using Yarn.
This will install Jest as a development dependency, meaning it's only needed for development and testing, not for your production application.
# What is a `jest.config.js` file and why do I need it?
A `jest.config.js` file is an optional JavaScript file located at the root of your project that allows you to customize Jest's behavior.
While Jest works with sensible defaults, this file becomes essential for more complex projects.
You need it to define things like your testing environment `jsdom` or `node`, how Jest should handle non-JavaScript files e.g., TypeScript, CSS, set up path aliases, configure code coverage, and many other advanced options.
It centralizes all your Jest settings, making them clear and maintainable.
# How do I specify the test environment in Jest?
You specify the test environment using the `testEnvironment` option in your `jest.config.js` file.
For browser-like testing e.g., React components, set `testEnvironment: 'jsdom'`. For Node.js backend code or pure utility functions, use `testEnvironment: 'node'`. For example:
// jest.config.js
module.exports = {
testEnvironment: 'jsdom', // or 'node'
}.
# How can I make Jest understand TypeScript or JSX?
To make Jest understand TypeScript or JSX, you need to configure a transformer.
For TypeScript, install `ts-jest` and `typescript`, then add it to your `transform` option:
transform: {
'^.+\\.ts|tsx$': 'ts-jest',
For JSX or modern JavaScript features not supported by your Node.js version, use `babel-jest` with Babel presets `@babel/preset-env`, `@babel/preset-react`:
'^.+\\.js|jsx$': 'babel-jest',
# What is `moduleNameMapper` used for in Jest?
`moduleNameMapper` is used to map import paths to specific files or modules. This is crucial for:
1. Resolving path aliases: If you use absolute imports like `import Component from 'src/components/MyComponent'`, `moduleNameMapper` tells Jest where `src` is.
2. Mocking static assets: It allows you to mock imports for non-JavaScript files like CSS, images, or fonts, preventing Jest from throwing errors when it encounters them during testing.
Example: `'^src/.*$': '<rootDir>/src/$1'` for aliases, and `'\\.css|less$': 'identity-obj-proxy'` for CSS.
# How do I speed up my Jest tests?
To speed up Jest tests, consider these optimizations:
1. `maxWorkers`: Adjust `maxWorkers` to utilize more CPU cores e.g., `'50%'` or a specific number like `4`.
2. Avoid `collectCoverage` locally: Only collect coverage when needed e.g., in CI using `jest --coverage`, as it adds significant overhead.
3. Use `jest --watch`: This interactive mode re-runs only relevant tests on file changes, drastically speeding up local development.
4. `testPathIgnorePatterns`: Ignore unnecessary directories and files from test discovery.
5. Caching: Ensure Jest's cache is enabled and preserved `cacheDirectory`.
# What is `setupFilesAfterEnv` and when should I use it?
`setupFilesAfterEnv` is an array of paths to modules that Jest will run once before each test file in the testing environment.
You should use it for global setup tasks that depend on the test environment being ready. Common uses include:
1. Importing and configuring testing libraries like `@testing-library/jest-dom` for custom matchers.
2. Globally mocking browser APIs e.g., `localStorage`, `fetch` for consistent test behavior.
3. Applying global polyfills.
# How do I debug Jest tests?
To debug Jest tests, you can use Node.js's inspector. Modify your `package.json` test script:
`"test": "node --inspect-brk ./node_modules/jest/bin/jest.js --runInBand"`
Then run `npm test`. Open your browser e.g., Chrome and navigate to `chrome://inspect` to attach the debugger.
The `--runInBand` flag forces all tests to run in a single process, simplifying debugging.
# How do I get code coverage reports with Jest?
To get code coverage reports, run Jest with the `--coverage` flag: `jest --coverage`. You can also configure `collectCoverage: true` in your `jest.config.js`, but it's generally better to use the CLI flag to avoid performance impact during regular development.
Jest will generate an HTML report in a `coverage/` directory by default, which you can view in your browser.
# Can Jest be used in a monorepo setup?
Yes, Jest is well-suited for monorepos through its `projects` configuration option.
You can define an array of separate Jest configurations, each tailored to a specific package within your monorepo.
This allows each package to have its own `testEnvironment`, `transform` rules, and `moduleNameMapper` settings, while still being runnable with a single `jest` command from the monorepo root.
# How do I configure Jest for a React project?
For a React project, you'll typically configure Jest with:
1. `testEnvironment: 'jsdom'` for browser-like testing.
2. `transform: {'^.+\\.js|jsx|ts|tsx$': 'babel-jest'}` and ensure Babel presets like `@babel/preset-react` are configured.
3. `setupFilesAfterEnv: ` to include `@testing-library/jest-dom`.
4. `moduleNameMapper` for any path aliases or mocking CSS/image imports.
# What are Jest presets and how do I use them?
Jest `presets` are pre-defined Jest configurations that simplify setup for common frameworks or environments e.g., `jest-preset-angular`, `ts-jest` can be used as a preset. They bundle common `transform`, `testEnvironment`, and other settings.
You use them by setting the `preset` option in your `jest.config.js`:
preset: 'ts-jest',
# What is the difference between `setupFiles` and `setupFilesAfterEnv`?
* `setupFiles`: Runs before the test environment is set up. Use this for polyfills or environment-specific global setup that must happen before Jest loads your test files or their environments.
* `setupFilesAfterEnv`: Runs after the test environment is set up, but before each test file. This is more common for configuring testing libraries like `@testing-library/jest-dom` or global mocks that depend on the environment being ready.
# How do I ignore specific files or directories from being tested by Jest?
You can ignore specific files or directories from Jest's test discovery using the `testPathIgnorePatterns` option in `jest.config.js`. Provide an array of glob patterns or regular expressions.
testPathIgnorePatterns: ,
# Can I run Jest tests in parallel?
Yes, Jest runs tests in parallel by default, using worker processes.
The `maxWorkers` option controls the number of parallel workers.
You can also use `test.concurrent` within a single test file to run individual tests concurrently within that file, controlled by `maxConcurrency`.
# How do I integrate Jest with my CI/CD pipeline?
Integrating Jest with CI/CD involves configuring your pipeline to run your test script e.g., `npm test` or `yarn test` as part of the build process.
You'll typically enable code coverage `--coverage` and potentially use Jest reporters like `jest-junit` to generate reports compatible with your CI/CD dashboard.
You might also set `coverageThreshold` to fail the build if coverage falls below a certain percentage.
# What is snapshot testing and how does Jest support it?
Snapshot testing is a Jest feature that captures a "snapshot" of a rendered UI component or serialized data structure and saves it as a file.
On subsequent test runs, Jest compares the new output to the saved snapshot.
If they don't match, it means a change occurred, and you're prompted to either accept the change update the snapshot or fix the code.
Jest supports it with the `.toMatchSnapshot` matcher.
# How do `globalSetup` and `globalTeardown` work?
`globalSetup` and `globalTeardown` are functions that run once before all test suites begin and once after all test suites have finished, respectively.
They are used for truly global, one-time operations like starting and stopping a test database, a mock server, or loading environment variables that affect the entire test run.
They run in their own Node.js processes, separate from the test workers.
# Why would I need a custom Jest resolver?
A custom Jest `resolver` is used for highly complex or non-standard module resolution scenarios that `moduleNameMapper` or `moduleDirectories` cannot handle.
This might be necessary in very specific monorepo setups, when dealing with unique package linking strategies, or when your project uses custom fields in `package.json` to define module entry points.
It allows you to programmatically define how Jest resolves module paths.
It's an advanced option and typically only used as a last resort.
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