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.