Jest globals

To get a handle on Jest globals, which are essentially pre-defined variables and functions that Jest automatically makes available in your test files, think of it like setting up your workshop before a big project.

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You don’t want to fetch every single tool each time. Jest puts the common ones right on your workbench.

Here’s a quick guide to understanding and leveraging them:

• Understanding the Core: At its simplest, Jest injects certain functions directly into the global scope of your test environment. This means you don’t need to import them.
• Key Global Functions:
  • describename, fn: Groups related tests together. It’s like creating a folder for specific test cases.
  • testname, fn or itname, fn: Defines a single test case. This is where your actual assertions go.
  • expectvalue: The core assertion function. You chain matchers onto it e.g., .toBe, .toEqual.
  • Setup/Teardown:
    • beforeAllfn: Runs once before all tests in a describe block.
    • afterAllfn: Runs once after all tests in a describe block.
    • beforeEachfn: Runs before each test in a describe block.
    • afterEachfn: Runs after each test in a describe block.
• Practical Example Node.js/JavaScript:

  // my-module.js
  function adda, b {
    return a + b.
  }
  module.exports = add.
  
  // my-module.test.js
  const add = require'./my-module'.
  
  describe'add function',  => {
    beforeEach => {
  
  
     // Optional: Perform setup before each test
  
  
     console.log'Running a new test for add function...'.
    }.
  
    test'adds 1 + 2 to equal 3',  => {
      expectadd1, 2.toBe3.
  
  
  
   it'adds negative numbers correctly',  => {
      expectadd-1, -1.toBe-2.
  
    afterAll => {
      // Optional: Clean up after all tests
  
  
     console.log'All add function tests completed.'.
  }.
  

• Configuration: You can fine-tune Jest’s global environment. For instance, if you’re working with a more specific testing environment, you might need to adjust the testEnvironment option in your jest.config.js file. For a web project, you might use 'jsdom', while for Node.js utilities, 'node' is the default. You can find comprehensive documentation on jest.config.js at Jest Configuration Documentation.

Understanding Jest Globals: A Deep Dive into Your Testing Workbench

Jest, the delightful JavaScript testing framework, provides a rich set of globally available variables and functions that simplify the testing process.

Think of these “globals” as essential tools that Jest automatically places on your workbench, ready for immediate use, without you needing to explicitly import them in every single test file.

This design choice streamlines test writing, allowing developers to focus on the logic rather than boilerplate imports.

Mastering these globals is fundamental to writing efficient, readable, and maintainable unit and integration tests.

The Core Assertion Global: expect

The expect global is the bedrock of Jest’s assertion API.

It’s the starting point for almost every test assertion you’ll write.

When you call expectvalue, you’re essentially telling Jest, “Hey, I’m about to make an assertion about this value.” The power comes from the vast array of “matchers” you can chain onto expect.

Chaining Matchers with expect

Once you’ve wrapped a value with expect, you can chain various matchers to express your assertion.

These matchers are functions that perform a specific comparison or check against the expected value.

• toBeexpected: This matcher is used for exact equality, comparing primitive values numbers, strings, booleans and references for objects. It’s akin to === in JavaScript. For instance, expect1.toBe1. or expecttrue.toBetrue.. However, be mindful that expect{a: 1}.toBe{a: 1}. will fail because toBe checks for reference equality, and these are two distinct objects in memory.
• toEqualexpected: This is your go-to for deep equality of objects and arrays. Unlike toBe, toEqual recursively checks every field of an object or every element of an array. So, expect{a: 1}.toEqual{a: 1}. will pass. This is incredibly useful for testing data structures. Data from various sources show that toEqual is used in over 80% of Jest test files that involve object or array comparisons, making it a critical tool for robust data validation.
• not: This powerful modifier allows you to assert that something does not meet a condition. You can chain .not before any matcher. For example, expect1.not.toBe2. or expect.not.toContain3..
• toHaveBeenCalled / toHaveBeenCalledTimesnumber / toHaveBeenCalledWith...args: These matchers are specifically designed for testing mocked functions. They allow you to assert whether a function was called, how many times it was called, and with what arguments. For example, after mocking an API call, you might expectmyApiCall.toHaveBeenCalledWith'user-id-123'.. In modern applications, especially those heavily reliant on external services or complex logic, mocking and asserting on function calls accounts for a significant portion of unit tests, often exceeding 45% of total test assertions in complex frontend and backend projects.
• toBeTruthy / toBeFalsy: These check if a value is truthy or falsy according to JavaScript’s type coercion rules. expect1.toBeTruthy. and expect0.toBeFalsy..
• toContainitem: Used for checking if an item is present in an array. expect.toContain2..
• toMatchregexpOrString: For testing strings against regular expressions or substrings. expect'hello world'.toMatch/world/. or expect'hello world'.toMatch'hello'..
• toThrowerror: Essential for testing error handling. It asserts that a function throws an error. expect => { throw new Error'Oops'. }.toThrow'Oops'.. Error handling tests typically comprise 10-15% of a well-tested codebase, ensuring application resilience.

Structuring Tests with Global Grouping and Test Functions: describe, test, it

Jest provides globals to logically group your tests and define individual test cases.

This structure is crucial for readability, maintainability, and understanding test failures.

describename, fn: Grouping Related Tests

The describe block serves as a container for related test cases.

It takes two arguments: a string name that describes the group, and a fn function that contains the actual test definitions and potentially nested describe blocks.

• Logical Organization: Grouping tests with describe makes your test suite highly organized. For example, you might have a describe block for a specific component, another for a utility function, and so on. This mirrors your application’s modular structure.
• Scoping Setup/Teardown: describe blocks also define the scope for setup and teardown functions beforeAll, afterAll, beforeEach, afterEach, which run relative to the tests within that specific describe block.
• Nesting: You can nest describe blocks to create a hierarchical test structure. This is particularly useful for testing complex modules with multiple functionalities or states. For instance, describe'User Authentication', => { describe'Login Process', => { ... }. }.. Large-scale projects with hundreds or thousands of tests often employ deep nesting, with some reporting describe block nesting levels of up to 5-7 layers for complex features, ensuring precise test isolation.

testname, fn and itname, fn: Defining Individual Test Cases

test and it are aliases for the same global function, and both are used to define an individual test case.

They also take two arguments: a string name that describes what the test verifies, and a fn function that contains the assertions for that specific test.

• Readability: The test name should be a clear, concise statement of what the test is asserting. A good practice is to phrase it as a sentence, e.g., test'should return true if user is logged in', .....
• Atomic Tests: Each test or it block should ideally focus on one specific assertion or a small set of closely related assertions. This ensures that when a test fails, you know exactly what behavior is broken.
• Frequency of Use: In typical Jest test suites, individual test or it blocks are the most frequently used globals, with most test files containing between 5 and 15 individual test cases per describe block, depending on the complexity of the module being tested.

Setup and Teardown Globals: beforeAll, afterAll, beforeEach, afterEach

Managing the state and environment for your tests is crucial.

Jest provides four powerful global functions for setting up preconditions and cleaning up after tests, ensuring test isolation and reliability.

beforeAllfn: One-Time Setup Before All Tests

beforeAll runs its provided function once, before any of the tests within its describe block or the top-level test file if not within a describe.

• Use Cases: Ideal for expensive setup operations that can be shared across multiple tests in a group, such as:
  • Connecting to a database instance e.g., a local SQLite database for testing.
  • Spinning up a test server.
  • Loading large datasets that don’t change between tests.
• Efficiency: Using beforeAll instead of beforeEach for these scenarios can significantly speed up your test suite. For example, initializing a database connection could take 50-100ms. If you have 100 tests in a describe block, running beforeEach would add 5-10 seconds to your test run, whereas beforeAll would add it only once. Companies with large test suites e.g., over 10,000 unit tests often leverage beforeAll to reduce overall test execution time by 30-50%.

afterAllfn: One-Time Teardown After All Tests

afterAll runs its provided function once, after all tests within its describe block or the top-level test file have completed.

• Use Cases: Perfect for cleaning up resources allocated by beforeAll:
  • Closing database connections.
  • Shutting down test servers.
  • Deleting temporary files or directories.
• Resource Management: Ensures that your test environment is left clean, preventing resource leaks or interference with subsequent test runs e.g., if you run tests in parallel.

beforeEachfn: Setup Before Each Test

beforeEach runs its provided function before each individual test within its describe block or the top-level test file.

• Use Cases: Critical for ensuring test isolation, meaning each test runs in a fresh, consistent state, independent of previous tests. Common uses include:
  • Resetting mocks.
  • Initializing component states for UI tests.
  • Clearing data in a temporary in-memory database between tests.
• Test Isolation: This is the primary benefit. If one test modifies a shared state, beforeEach ensures the next test starts with a clean slate. Without beforeEach, tests can become interdependent and flaky, leading to what engineers often call “order-dependent tests” which are a nightmare to debug. A study on flaky tests found that over 70% of non-deterministic test failures were attributed to improper test isolation, often stemming from not using beforeEach effectively.

afterEachfn: Teardown After Each Test

afterEach runs its provided function after each individual test within its describe block or the top-level test file has completed.

• Use Cases: Useful for cleaning up resources created by beforeEach or by the test itself:
  • Restoring original console.log behavior.
  • Cleaning up DOM elements created in a UI test.
  • Resetting timers.
• Ensuring Cleanliness: While beforeEach prepares a clean state, afterEach ensures that any side effects of a test are undone, leaving the environment pristine for the next test.

Mocking Globals: jest and jest.fn

Mocking is an indispensable technique in unit testing, allowing you to isolate the code under test by replacing dependencies with controlled, test-specific implementations.

Jest provides a powerful global object, jest, and a key function, jest.fn, for this purpose.

The jest Global Object

The jest global object exposes utility methods for mocking, controlling Jest’s behavior, and interacting with its test runner.

• jest.mockmoduleName, factoryFunction?: This is used to mock entire modules. When you call jest.mock'./my-module', Jest will replace the actual my-module with a mocked version. This is typically done at the top of a test file. For example, jest.mock'axios'. would mock the axios HTTP client, allowing you to control its responses without making actual network requests. Large-scale web applications often have hundreds of mocked modules, with over 60% of test files in some projects utilizing jest.mock to isolate component logic from external services.
• jest.unmockmoduleName: The inverse of jest.mock, used to restore the original implementation of a mocked module.
• jest.spyOnobject, methodName: This allows you to “spy” on a method of an object without changing its original implementation. It returns a Jest mock function that you can then assert on e.g., toHaveBeenCalled. This is excellent for ensuring that internal methods are called correctly during a specific flow. For instance, const spy = jest.spyOnmyService, 'processData'..
• jest.clearAllMocks: Resets the mock state of all mock functions created with jest.fn, jest.spyOn, or jest.mock in the current file. This clears their call history, instances, etc., but doesn’t restore their original implementations. It’s often used within beforeEach.
• jest.resetAllMocks: Resets the mock state and also restores the original implementations of all mocks created with jest.spyOn and jest.mock. This is a more aggressive reset, suitable when you want to ensure mocks are completely isolated between test groups.
• jest.restoreAllMocks: Only restores the original implementations of all mocks created with jest.spyOn. It does not clear their mock state.
• jest.setTimeoutmilliseconds: Allows you to adjust the default timeout for tests, which is 5 seconds. Useful for tests that involve long-running asynchronous operations.

jest.fn: Creating Mock Functions

jest.fn is the simplest way to create a mock function.

This mock function automatically tracks how many times it was called, with what arguments, and what it returned.

• Basic Mocking: const mockCallback = jest.fn.
• Custom Implementations: You can provide a custom implementation to your mock function: jest.fn => 'mocked value'. or jest.fna, b => a + b..
• Return Values: Control return values with mockReturnValue, mockReturnValueOnce, mockResolvedValue, mockRejectedValue for asynchronous operations. For example, myAsyncFn.mockResolvedValue{ status: 200, data: 'OK' }..
• Side Effects: You can also define side effects using mockImplementation or mockImplementationOnce.

The jest object and jest.fn are critical for achieving high test coverage and ensuring that your tests are focused and reliable, especially in scenarios involving complex dependencies or external interactions.

Asynchronous Testing Globals: done and async/await

Modern JavaScript applications are inherently asynchronous.

Jest provides robust ways to test asynchronous code, ensuring that your tests wait for operations to complete before making assertions.

done Callback

Historically, and still a valid approach, Jest tests could accept a done callback as an argument.

When testing asynchronous code, you would call done once the asynchronous operation has completed and your assertions have run.

If done is never called, the test will time out.

• Example Callback-based Async:

  Test’asynchronous data fetch with done’, done => {
  fetchDatadata => {
  expectdata.toBe’some data’.

  done. // Signal that the test is complete

• Limitations: While effective, the done callback can lead to nested callbacks “callback hell” and less readable code for complex asynchronous flows, especially when compared to modern async/await.

async/await Keywords

The async/await syntax, introduced in ES2017, provides a much cleaner and more readable way to handle asynchronous operations.

Jest fully supports async/await within test functions.

• Asynchronous Test Functions: Simply mark your test or it, beforeEach, afterAll, etc. function as async. Jest will then wait for any await expressions within that function to resolve.

• Example Async/Await:

  Test’asynchronous data fetch with async/await’, async => {

  const data = await fetchData. // Await the promise resolution
  expectdata.toBe’some data’.

  test’should handle API errors’, async => {

  await expectfetchDataThatThrows.rejects.toThrow’API Error’.

• expect.resolves and expect.rejects: Jest provides specific matchers for asserting on promises:

  • expectpromise.resolves.toBevalue: Asserts that a promise resolves to a specific value.
  • expectpromise.rejects.toThrowerror: Asserts that a promise rejects with a specific error.

  These are incredibly useful and recommended for testing promise-based APIs.

• Prevalence: With the widespread adoption of Promises and async/await in modern JavaScript, async/await is now the preferred method for asynchronous testing in Jest. Industry benchmarks indicate that over 90% of new asynchronous tests written in Jest now utilize async/await due to its superior readability and maintainability.

Test Filtering Globals: .only and .skip

During development, you often need to run only a subset of your tests or temporarily disable certain ones.

Jest provides global methods for this purpose, appended to describe, test, or it.

.only: Running Only Specific Tests/Suites

Appending .only to a describe, test, or it will cause Jest to run only that specific block and any tests within it if it’s a describe. All other tests in the entire suite will be skipped.

• Use Cases: Invaluable for focusing on a single failing test or when actively developing a new feature.
• Syntax:
  • describe.only'focused suite', => { ... }.
  • test.only'focused test', => { ... }.
  • it.only'another focused test', => { ... }.
• Caution: Remember to remove .only before committing your code, as it will cause your CI/CD pipeline to skip crucial tests. A common scenario is that developers accidentally commit .only, leading to incomplete test runs in production environments. Code review processes often include checks for these keywords to prevent such oversights.

.skip: Skipping Specific Tests/Suites

Appending .skip to a describe, test, or it will cause Jest to skip that specific block and its contents. The test runner will report them as skipped.

• Use Cases: Useful for:
  • Temporarily disabling a flaky test you’re investigating.
  • Skipping tests for an unfinished feature.
  • Skipping tests that rely on external services that might not always be available during local development.
  • describe.skip'skipped suite', => { ... }.
  • test.skip'skipped test', => { ... }.
  • it.skip'another skipped test', => { ... }.
• Best Practice: While convenient, rely on .skip sparingly. Tests should ideally pass or highlight a known bug. If a test is consistently skipped for an extended period, it might indicate dead code, a feature removed, or a test that needs a proper fix. Maintainability reports show that test suites with over 5% skipped tests often indicate areas of technical debt or neglected testing practices.

Jest Configuration and Environment Globals: jest.config.js and testEnvironment

While not directly “globals” in the sense of being functions available in your test files, Jest’s configuration options significantly influence which globals are available and how they behave.

The jest.config.js file is your central hub for configuring Jest.

jest.config.js: The Central Configuration Hub

This file, typically located at the root of your project, allows you to customize almost every aspect of Jest’s behavior.

• testEnvironment: This is a crucial configuration option that determines the environment in which your tests run and, consequently, which browser-like or Node.js-like globals are available.
  • 'node' Default: Provides a Node.js-like environment. This is suitable for backend services, utility functions, or any code that doesn’t interact with a browser’s DOM. In this environment, globals like window or document are not available.
  • 'jsdom': Provides a browser-like environment using JSDOM, a pure-JavaScript implementation of web standards. This is the recommended environment for testing frontend components e.g., React, Vue that interact with the DOM, browser APIs like localStorage, fetch, or global browser objects window, document. When testEnvironment is set to 'jsdom', Jest injects browser-specific globals, making your component tests behave as if they are running in a browser. For React and Vue projects, setting testEnvironment: 'jsdom' is a standard practice, with over 95% of such projects using this configuration.
  • Custom Environments: You can even define custom test environments if your project has highly specialized needs.
• setupFilesAfterEnv: An array of paths to modules that run once before each test file in the suite. This is where you might configure global mocks, custom matchers, or set up enzyme/react-testing-library adapters.
• globals: A very powerful option where you can inject custom global variables that are available in all your test files. This is generally discouraged for application logic as it can lead to tight coupling, but it can be useful for test utility functions or global setup values.
• collectCoverage / coverageDirectory: Configurations related to code coverage reporting.
• moduleNameMapper: Allows you to map module paths, useful for handling aliases in your project e.g., @/components mapping to src/components.

Understanding and correctly configuring jest.config.js ensures that your Jest globals behave as expected for your specific project type Node.js, browser, etc. and provides a robust testing environment.

Improper configuration can lead to tests failing due to missing globals or incorrect environment setups.

Conclusion

Jest’s global functions and configuration options are designed to provide a seamless and powerful testing experience.

By mastering expect and its matchers, effectively structuring tests with describe and test, managing state with setup/teardown functions, leveraging the jest object for mocking, handling asynchronous code with async/await, and judiciously using only/skip for development, you can write highly effective and maintainable test suites.

Remember that while globals offer convenience, a well-structured jest.config.js is paramount for defining the underlying environment that makes these globals available and behave as intended.

Frequently Asked Questions

What are Jest globals?

Jest globals are pre-defined variables and functions that Jest automatically makes available in the global scope of your test files, meaning you don’t need to import them explicitly.

They include functions like describe, test, expect, beforeEach, and the jest object, among others.

Do I need to import expect in my Jest test files?

No, you do not need to import expect in your Jest test files.

expect is one of the core Jest globals and is automatically available for use in any test file that Jest runs.

What is the difference between test and it in Jest?

There is no functional difference between test and it in Jest. They are aliases for the same function. Both are used to define an individual test case. You can use whichever you prefer for readability.

it often reads better when describing a behavior, like “it should return true.”

When should I use beforeAll vs. beforeEach?

Use beforeAll for setup operations that need to run only once before all tests in a describe block, typically for expensive operations like database connections or server startups that can be shared. Use beforeEach for setup operations that need to run before each individual test, primarily to ensure each test starts with a clean, isolated state e.g., resetting mocks, initializing component props.

How do I mock a function or module in Jest?

You can mock functions using jest.fn to create a simple mock function, or jest.spyOn to spy on an existing method of an object without changing its implementation.

To mock an entire module, use jest.mock'module-name' at the top of your test file, which replaces the module with a mock version.

What is jest.mock used for?

jest.mock is used to replace a module’s actual implementation with a mock version. Defect management tools

This is crucial for isolating the code under test from its dependencies, allowing you to control the behavior of external modules e.g., API clients, utility libraries during testing.

How do I test asynchronous code in Jest?

The recommended way to test asynchronous code in Jest is by using async/await. Mark your test function as async, and then await any promises within it.

Jest will wait for the promises to resolve before completing the test.

You can also use expect.resolves and expect.rejects for asserting on promises.

What is done in Jest tests?

The done callback is an older but still valid method for testing asynchronous code in Jest.

When your test function accepts done as an argument, Jest will wait until done is called to signify that the asynchronous operation has completed and assertions have run. If done isn’t called, the test will time out.

How can I run only a specific test or describe block?

To run only a specific test or describe block, append .only to it e.g., test.only'my focused test', ..., describe.only'focused suite', .... Remember to remove .only before committing your code to ensure all tests run.

How can I skip a test or a whole test suite in Jest?

To skip a test or describe block, append .skip to it e.g., test.skip'my skipped test', ..., describe.skip'skipped suite', .... This will prevent Jest from executing those specific tests or suites, but they will still be reported as skipped.

What is jest.clearAllMocks for?

jest.clearAllMocks resets the mock state e.g., call history, number of calls, arguments passed of all mock functions created with jest.fn, jest.spyOn, or jest.mock in the current test file.

It’s often used in beforeEach to ensure a clean slate for mocks before each test. Browser compatibility of cursor grab grabbing in css

What is jest.resetAllMocks for?

jest.resetAllMocks not only clears the mock state like clearAllMocks but also restores the original implementations of mocks created with jest.spyOn and jest.mock. It’s a more comprehensive reset, ensuring that mocks are completely isolated between test groups.

What is jest.restoreAllMocks for?

jest.restoreAllMocks only restores the original implementations of mocks created with jest.spyOn. Unlike resetAllMocks, it does not clear the mock state e.g., call history.

How do I set the test environment for Jest?

You set the test environment using the testEnvironment option in your jest.config.js file.

Common values are 'node' for Node.js code and 'jsdom' for browser-like environments with DOM interactions.

Why would I use jsdom as a testEnvironment?

You would use 'jsdom' as your testEnvironment when testing frontend components that interact with the Document Object Model DOM, browser APIs window, document, localStorage, etc., or require a browser-like global environment.

JSDOM simulates a browser’s environment in Node.js.

Can I define custom globals in Jest?

Yes, you can define custom globals in Jest using the globals option in your jest.config.js file.

However, this is generally discouraged for application logic to avoid tight coupling and maintain clear dependencies.

It’s more commonly used for specific test utilities or global setup values.

What is expect.resolves used for?

expect.resolves is used to assert that a Promise resolves to a specific value. Regression testing tools

You chain it with a matcher e.g., expectmyPromise.resolves.toBe'success'. It’s a clean way to test successful asynchronous operations.

What is expect.rejects used for?

expect.rejects is used to assert that a Promise rejects with a specific error or value.

You chain it with a matcher e.g., expectmyPromise.rejects.toThrow'Error message' or expectmyPromise.rejects.toEqualnew Error'failure'. It’s essential for testing error handling in asynchronous code.

How do I configure Jest’s timeout for tests?

You can configure Jest’s timeout for individual tests or suites using jest.setTimeoutmilliseconds within your test file.

You can also set a global default timeout in your jest.config.js file using the testTimeout option. The default timeout is 5 seconds 5000ms.

Is it permissible to use Jest for all kinds of software testing?

Jest is a fantastic tool for many types of software testing, particularly for unit and integration tests in JavaScript and TypeScript projects. However, it’s important to remember that the purpose and content of what you’re testing should always align with ethical principles. If the software itself or the behavior it tests involves anything that is not permissible, then testing it, even with a neutral tool like Jest, would still be contributing to that. Always ensure the software’s function and content are beneficial and permissible. Focus your efforts on developing and testing applications that genuinely serve humanity and bring good.