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Jest vs Karma: What are the differences?

Jest and Karma are popular JavaScript testing frameworks that enable developers to write and execute tests. Let's explore the key differences between Jest and Karma in more detail:

  1. Design Philosophy: Jest, developed by Facebook, prioritizes a seamless and intuitive testing experience for JavaScript applications. It offers built-in features like test runners, assertions, code coverage, and mocking. Karma acts as a middleware test runner, focusing on cross-browser testing and providing a flexible platform for running tests in different environments.

  2. Features: Jest provides a comprehensive feature set including a built-in test runner, mocking, snapshot testing, code coverage reporting, and a powerful assertion library. It supports parallel test execution for faster results. Karma integrates with testing frameworks like Jasmine, Mocha, and QUnit, offering browser automation, live reloading, test result reporting, and compatibility with diverse browsers and devices.

  3. Ecosystem Integration: Jest tightly integrates with the React ecosystem, offering additional features tailored for testing React applications such as snapshot testing. It supports frameworks like Enzyme and React Testing Library, making it popular among React developers. Karma is framework-agnostic, compatible with various JavaScript frameworks and libraries, and commonly used with Angular, Vue.js, and plain JavaScript projects.

  4. Configuration and Setup: Jest aims to provide a zero-configuration experience by offering sensible defaults that work out-of-the-box for most projects. It requires minimal setup and can be easily integrated into JavaScript projects. Karma, on the other hand, requires more configuration and setup as it acts as a middleware between testing frameworks and browsers. It needs additional configuration files to specify the testing framework, browsers, preprocessors, and other settings.

  5. Browser Compatibility: Jest runs tests in a Node.js environment and does not require an actual browser to execute tests. It uses jsdom, a JavaScript implementation of the DOM, to simulate the browser environment. Karma, on the other hand, allows running tests in actual browsers, enabling cross-browser testing and ensuring compatibility across different browser environments.

  6. Community and Support: Both Jest and Karma have active and vibrant communities, with extensive documentation and support resources available. Jest, being developed by Facebook, has a large user base and benefits from the contributions and support of the React community. Karma, being a popular test runner, also has a strong community and is widely adopted across various JavaScript frameworks.

In summary, Jest and Karma are JavaScript testing frameworks with distinct design philosophies and features. Jest offers a comprehensive and intuitive testing experience with built-in features, focusing on seamless integration with the React ecosystem. Karma, on the other hand, acts as a test runner that enables cross-browser testing and works well with a variety of JavaScript frameworks.

Decisions about Jest and Karma
Shared insights

As we all know testing is an important part of any application. To assist with our testing we are going to use both Cypress and Jest. We feel these tools complement each other and will help us get good coverage of our code. We will use Cypress for our end to end testing as we've found it quite user friendly. Jest will be used for our unit tests because we've seen how many larger companies use it with great success.

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Postman will be used to do integration testing with the backend API we create. It offers a clean interface to create many requests, and you can even organize these requests into collections. It helps to test the backend API first to make sure it's working before using it in the front-end. Jest can also be used for testing and is already embedded into React. Not only does it offer unit testing support in javascript, it can also do snapshot testing for the front-end to make sure components are rendering correctly. Enzyme is complementary to Jest and offers more functions such as shallow rendering. UnitTest will be used for Python testing as it is simple, has a lot of functionality and already built in with python. Sentry will be used for keeping track of errors as it is also easily integratable with Heroku because they offer it as an add-on. LogDNA will be used for tracking logs which are not errors and is also a Heroku add-on. Its good to have a separate service to record logs, monitor, track and even fix errors in real-time so our application can run more smoothly.

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Pros of Jest
Pros of Karma
  • 36
    Open source
  • 32
    Mock by default makes testing much simpler
  • 23
    Testing React Native Apps
  • 20
    Parallel test running
  • 16
  • 13
    Bundled with JSDOM to enable DOM testing
  • 8
    Mock by default screws up your classes, breaking tests
  • 7
    Out of the box code coverage
  • 7
    Promise support
  • 6
    One stop shop for unit testing
  • 3
    Great documentation
  • 2
    Assert Library Included
  • 1
    Built in watch option with interactive filtering menu
  • 1
    Preset support
  • 0
    Can be used for BDD
  • 0
  • 61
    Test Runner
  • 35
    Open source
  • 27
    Continuous Integration
  • 22
    Great for running tests
  • 18
    Test on Real Devices
  • 11
    Backed by google
  • 5
    Easy Debugging
  • 2
    Remote Control

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Cons of Jest
Cons of Karma
  • 4
  • 4
    Ambiguous configuration
  • 3
  • 2
    Many bugs still not fixed months/years after reporting
  • 2
    Multiple error messages for same error
  • 2
    Difficult to run single test/describe/file
  • 2
  • 2
  • 1
    BeforeAll timing out makes all passing tests fail
  • 1
  • 1
    Reporter is too general
  • 1
  • 1
    Bad docs
  • 1
    Still does't support .mjs files natively
  • 1
    Can't fail beforeAll to abort tests
  • 0
    Interaction with watch mode on terminal
  • 1
    Slow, because tests are run in a real browser
  • 1
    Requires the use of hacks to find tests dynamically

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What is Jest?

Jest provides you with multiple layers on top of Jasmine.

What is Karma?

Karma is not a testing framework, nor an assertion library. Karma just launches a HTTP server, and generates the test runner HTML file you probably already know from your favourite testing framework. So for testing purposes you can use pretty much anything you like.

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What companies use Jest?
What companies use Karma?
See which teams inside your own company are using Jest or Karma.
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What tools integrate with Karma?

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What are some alternatives to Jest and Karma?
Mocha is a feature-rich JavaScript test framework running on node.js and the browser, making asynchronous testing simple and fun. Mocha tests run serially, allowing for flexible and accurate reporting, while mapping uncaught exceptions to the correct test cases.
Selenium automates browsers. That's it! What you do with that power is entirely up to you. Primarily, it is for automating web applications for testing purposes, but is certainly not limited to just that. Boring web-based administration tasks can (and should!) also be automated as well.
Even though JavaScript is single-threaded, IO in Node.js can happen in parallel due to its async nature. AVA takes advantage of this and runs your tests concurrently, which is especially beneficial for IO heavy tests. In addition, test files are run in parallel as separate processes, giving you even better performance and an isolated environment for each test file.
Enzyme is a JavaScript Testing utility for React that makes it easier to assert, manipulate, and traverse your React Components' output.
Jasmine is a Behavior Driven Development testing framework for JavaScript. It does not rely on browsers, DOM, or any JavaScript framework. Thus it's suited for websites, Node.js projects, or anywhere that JavaScript can run.
See all alternatives