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Jest vs Karma: What are the differences?

Karma is a JavaScript test runner. It helps run the testing of the frontend in a real browser, running the test against the production build in a real browser and can help find discrepancies across different browsers. Jest is a must consideration for React users. It is efficient and stable; Jest focusses on the JavaScript and not the static assets or CSS. It supports the ability to write custom transformers that specify how assets transform during the build process. Jest also has test change detection and parallelism. And lastly, has robust logging and stack trace reporting.

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 are some alternatives to Jest and Karma?
Mocha
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
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.
AVA
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.
Jasmine
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.
Cypress
Cypress is a front end automated testing application created for the modern web. Cypress is built on a new architecture and runs in the same run-loop as the application being tested. As a result Cypress provides better, faster, and more reliable testing for anything that runs in a browser. Cypress works on any front-end framework or website.
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Decisions about Jest and Karma
Dschinkel Schinkel
Dschinkel Schinkel
Mocha
Mocha
Jest
Jest
JavaScript
JavaScript
React
React
Enzyme
Enzyme
#Tdd
#Bdd
#Testdrivendevelopment

I use both mocha and Jest because:

  • I don't care whether teams use Jest or Mocha. But jest is way too overhyped. Most devs are writing integration tests and think that it's so much better but frankly I don't write integration tests as the way to get both design feedback and confidence when I code. I adhere to the test pyramid, not ice cream cone or the dumb "trophy"

  • I TDD, so I only ever use the "API" of test frameworks. I don't do a lot of integration tests for TDD and all the bells and whistles Jest provides you from the command-line I just don't need. And I certainly do not care about or touch Jest Snapshots, I despise them

  • My tests are fast enough because I write isolated tests with TDD, so I don't run into performance issues. Example: I write my tests in a way that I can run 300 tests in literally 1 second with mocha. So the Jest ability to pinpoint and only run those tests which are affected by code changes. I want to run all of them every time when I TDD. It's a different mindset when you TDD

  • I also mainly code in IntelliJ or WebStorm because I feel the tools in that IDE far surpass VSCode and I also love running the test UI runner in it vs. lousy command-line

  • I feel both mocha and Jest read just fine in terms of code readability. Jest might have shorter assertion syntax but I don't really care. I just care that I can read the damn test and my tests are written well and my test descriptions, as well as the code itself including constants represent business language, not technical. I care most about BDD, clean code, 4 rules of simple design, and SOLID

  • I don't like using mock frameworks so no I don't use Jest's Mocking framework. I don't have to mock a lot in my tests due to the nature of how I strive to code...I keep my design simple and modular using principals such as clean code and 4 rules of simple design. If I must mock, I create very simple custom mocks with JS

  • On the contrary to the belief that integration tests and mount are the way to go (this belief drives me absolutely crazy, especially Dodd's promoting that), I TDD with shallow & enzyme. My tests are simple. My design is driven by my tests and my tests give me quick and useful feedback. I have a course I'm working on coming out soon on TDD with React to show you how to truly test the FE and why the ice cream cone and trophy suck (you're being scammed people). Watch for that here: https://twitter.com/DaveSchinkel/status/1062267649235791873

Don't forget to upvote this post!

Mocha Jest JavaScript React @jsdom Enzyme #tdd #bdd #testdrivendevelopment

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Russel Werner
Russel Werner
Lead Engineer at StackShare | 6 upvotes 62.4K views
atStackShareStackShare
Jest
Jest
Enzyme
Enzyme
React Storybook
React Storybook
Happo.io
Happo.io
Percy
Percy

We use Jest because when we rebooted our "front end" stack earlier last year, we need to have a testing solution (we didn't have any front-end tests before that!). Jest is fast and convenient and it has plenty of community support behind it. It let's us run our unit tests with Enzyme and snapshot tests.

This is an area that we are constantly reviewing to see what can be improved, both in terms of developer needs, accuracy, test maintainability, and coverage.

I'm currently exploring using React Storybook to be the record of snapshot tests and using some online services, such as Happo.io and Percy in our CI pipeline.

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Scott Mebberson
Scott Mebberson
CTO / Chief Architect at Idearium | 2 upvotes 21.8K views
Mocha
Mocha
Jest
Jest

We used to Mocha for as our primary Node.js test framework. We've now switched to Jest and haven't looked back.

Jest is faster and requires less setup and configuration. The Mocha API and eco-system is vast and verified, but that also brings complexity.

It you want to get in, write tests, execute them and get out, try Jest 馃榾

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Gustavo Mu帽oz
Gustavo Mu帽oz
Web UI Developer at Globant | 1 upvotes 1.9K views
Jest
Jest
Enzyme
Enzyme

I really enjoy using Jest as my testing framework. I also use Enzyme to complement, and both together are amazing. Jest is fast and easy to use, It has all you need together under the same tool, and it's pretty easy to create all kind of test, even asynchronous ones. I was responsible for implant it in our company projects, and it was the best decision for testing.

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Jack Graves
Jack Graves
Head of Product Development at Automation Consultants | 3 upvotes 31.1K views
atAutomation ConsultantsAutomation Consultants
JUnit
JUnit
Jest
Jest
Apache JMeter
Apache JMeter
Mocha
Mocha

We use JUnit and Jest to perform the bulk of our automated test scenarios, with additional work with Apache JMeter for performance testing - for example, the Atlassian Data Center compliance testing is performed with JMeter. Jest provides testing for the React interfaces, which make up the backend of our App offerings. JUnit is used for Unit Testing our Server-based Apps. Mocha is another tool we use.

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Robert Zuber
Robert Zuber
CTO at CircleCI | 16 upvotes 304.4K views
atCircleCICircleCI
Next.js
Next.js
React
React
Storybook
Storybook
TypeScript
TypeScript
Emotion
Emotion
GraphQL
GraphQL
Apollo
Apollo
Jest
Jest
Percy
Percy
Cypress
Cypress

We are in the process of adopting Next.js as our React framework and using Storybook to help build our React components in isolation. This new part of our frontend is written in TypeScript, and we use Emotion for CSS/styling. For delivering data, we use GraphQL and Apollo. Jest, Percy, and Cypress are used for testing.

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Interest over time
Reviews of Jest and Karma
Avatar of cristiangiagante
.Net Developer at Hexacta
Review ofJestJest

I'm using Jest for 3 months in a Vue JS project . I need to use a lot of custom search of related topics in jest docs because it's not clear. The examples are very poor too.

How developers use Jest and Karma
Avatar of Volkan 脰z莽elik
Volkan 脰z莽elik uses JestJest

Jest is my unit-testing tool of choice.

Almost all unit testing suites (Mocha, Jasmine, etc.) are more or less the same.

The main advantage I guess, is that it integrates pretty well with React and Enzyme.

Avatar of Riderman De Sousa Barbosa
Riderman De Sousa Barbosa uses KarmaKarma

All services, directives and controllers from angular are tested using Karma.

Avatar of Ron Apelbaum
Ron Apelbaum uses KarmaKarma

I use Karma as a test runner for javascript unit tests

Avatar of Giovanni Candido da Silva
Giovanni Candido da Silva uses KarmaKarma

Run unit and integration tests on real browsers

Avatar of Glib Ischenko
Glib Ischenko uses KarmaKarma

Running Unit tests for Angular UI

Avatar of Typeform
Typeform uses KarmaKarma

Testing frontend of admin part

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