Jest is a powerful JavaScript testing framework that simplifies unit testing, integration testing, and beyond. Here are its core features:

1. Mock Functions 🧠

Jest allows you to create mock functions to simulate behavior. 

jest.fn().mockReturnValue("mocked");

Use jest.spyOn() for spying on existing functions.

Mock_Functions

2. Snapshot Testing 📸

Compare UI outputs against predefined snapshots. 

expect(component).toMatchSnapshot();

Snapshots are auto-generated and stored in __snapshots__ folder.

Snapshot_Testing

3. Asynchronous Code Support ⏱️

Test async functions with async/await or done callbacks. 

test("async test", async () => {
  await expect(fetchData()).resolves.toBe("success");
});

Use findBy or wait for React components.

Asynchronous_Code

4. Test Coverage 📊

Analyze code coverage with built-in reporter: 

npm test -- --coverage

View results in coverage/ directory.

Test_Coverage

5. Assertions & Matchers ✅

Rich assertion library:

  • toBe() for strict equality
  • toContain() for array inclusion
  • toThrow() for error checking
  • toHaveLength() for string/array length

Next Steps 🚀

Explore how to set up Jest in your project or dive into advanced configurations.