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Chai vs Jest: What are the differences?

Chai and Jest are both popular JavaScript testing frameworks used for writing unit tests. Let's explore the key differences between them.

  1. Syntax: Chai allows developers to choose from multiple assertion styles, including Should, Expect, and Assert, giving them flexibility in writing assertions in a way that feels comfortable. On the other hand, Jest has a built-in expectation library, using a more opinionated syntax where assertions are written with the "expect" keyword.

  2. Mocks and Spies: In Chai, developers need to use separate libraries like Sinon or Testdouble to create mocks and spies for stubbing or spying on function calls and verification purposes. However, Jest has built-in functions like "jest.spyOn" and "jest.fn" that can be used directly without any external dependencies, providing a more seamless mocking and spying experience.

  3. Configuration: Chai does not come with a built-in test runner or configuration. Developers have the freedom to choose their preferred test runner (e.g., Mocha, Jasmine) and configure it as per their needs. In contrast, Jest provides a fully-featured test runner and configuration out of the box, reducing setup time and the need for additional configurations.

  4. Performance: Chai can be slower compared to Jest for larger test suites due to its assertion style flexibility and need for external libraries for functionalities like mocking and spying. Jest, on the other hand, is designed to prioritize performance, utilizing parallelism and smart test skipping techniques, resulting in faster running tests.

  5. Snapshot Testing: Jest has built-in support for snapshot testing, allowing developers to capture the rendered output of components and compare it with the stored snapshot to detect any unintended changes. Chai does not have native support for snapshot testing, requiring developers to use extra libraries like Chai-enzyme or Enzyme to achieve similar functionality.

  6. Test Coverage Analysis: Jest provides built-in tools for generating code coverage reports, making it easier for developers to track the amount of code covered by their tests. Chai itself does not have native support for code coverage analysis, requiring integration with additional tools or libraries to achieve the same level of coverage tracking.

In summary, Chai provides flexibility in assertion style and allows developers to choose their preferred test runner, whereas Jest offers a more opinionated syntax and comes with a built-in test runner along with features like snapshot testing and code coverage analysis.

Decisions about Chai and Jest
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CypressCypressJestJest

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 Chai
Pros of Jest
    Be the first to leave a pro
    • 36
      Open source
    • 32
      Mock by default makes testing much simpler
    • 23
      Testing React Native Apps
    • 20
      Parallel test running
    • 16
      Fast
    • 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
      Karma

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    Cons of Chai
    Cons of Jest
      Be the first to leave a con
      • 4
        Documentation
      • 4
        Ambiguous configuration
      • 3
        Difficult
      • 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
        Ambiguous
      • 2
        Bugged
      • 1
        BeforeAll timing out makes all passing tests fail
      • 1
        Slow
      • 1
        Reporter is too general
      • 1
        Unstable
      • 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

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      What is Chai?

      It is a BDD / TDD assertion library for node and the browser that can be delightfully paired with any javascript testing framework. It has several interfaces that allow the developer to choose the most comfortable. The chain-capable BDD styles provide an expressive language & readable style, while the TDD assert style provides a more classical feel.

      What is Jest?

      Jest provides you with multiple layers on top of Jasmine.

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      See which teams inside your own company are using Chai or Jest.
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