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Published on 4/24/2025
Testing React
Rules
## Rules for Testing JavaScript and React Applications
Drawing from the content and philosophy of [TestingJavaScript.com][1][3][2], as well as established industry best practices, here are clear cursor rules to guide coding agents on how to test JavaScript and React applications effectively.
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### **General JavaScript Testing Rules**
- **Test Behavior, Not Implementation**
- Write tests that focus on the observable behavior of your code, not its internal implementation or private variables. This ensures your tests remain robust even as the code evolves[10][7].
- **Keep Tests Small and Focused**
- Each test should cover a single scenario or behavior. Avoid writing tests that try to verify multiple things at once[4].
- **Use Descriptive Test Names**
- Name your tests so that anyone can understand what is being tested and why. Good names improve test readability and maintainability[4][8].
- **Organize Tests by Type and Feature**
- Group related tests together (unit, integration, end-to-end) and structure them to reflect your application's features[4].
- **Mock External Dependencies Only When Necessary**
- Use mocks and stubs to isolate the unit under test, but avoid over-mocking. Prefer real implementations when possible to keep tests meaningful and less brittle[8][10].
- **Maintain Test Code Quality**
- Treat test code as production code: keep it clean, organized, and up-to-date[4][6].
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### **React Application Testing Rules**
- **Test Components as Users Would Use Them**
- Use tools like React Testing Library to test the behavior of components from the user's perspective, interacting with the DOM as a user would. Avoid testing internal component state or methods directly[5][7][9].
- **Prefer Queries Based on Accessible Attributes**
- When selecting elements in tests, use queries that reflect how users interact (e.g., by label text, role, or placeholder) rather than brittle selectors like CSS classes or IDs[9].
- **Use the Arrange-Act-Assert (AAA) Pattern**
- Structure each test to first *arrange* the environment, then *act* (perform user interaction or trigger logic), and finally *assert* the expected outcome[7].
- **Write Unit, Integration, and End-to-End Tests**
- Unit tests: Test individual functions or components in isolation.
- Integration tests: Test how multiple components or modules work together.
- End-to-end tests: Simulate real user workflows in a browser environment[5].
- **Use Jest and React Testing Library for Unit and Component Tests**
- Jest is the recommended test runner and assertion library for React apps. Use React Testing Library for rendering and interacting with components in tests[5].
- **Automate and Run Tests Regularly**
- Integrate tests into your development workflow and CI/CD pipelines to catch regressions early[4][6].
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### **Common Anti-Patterns to Avoid**
- **Don’t Test Private/Internal Implementation Details**
- Testing internal logic makes tests brittle and harder to maintain[10][7].
- **Don’t Overuse Mocks or Test Helpers**
- Excessive mocking can make tests less reliable and harder to understand[8][10].
- **Don’t Rely on Unstable Selectors**
- Avoid querying elements by CSS classes or IDs that may change with design updates; use data attributes or accessible queries instead[9].
- USE VITEST INSTEAD OF JEST ALWAYS
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### **Summary Table**
| Rule | Rationale/Example |
|-----------------------------------------------|-----------------------------------------------------------------------------------|
| Test behavior, not implementation | Focus on what the user sees, not how it’s built[10][7] |
| Use user-centric queries in React tests | `getByText`, `getByRole` over `.className`[9] |
| Keep tests small, focused, and descriptive | Easier to maintain and debug[4][8] |
| Use Arrange-Act-Assert pattern | Improves clarity and structure[7] |
| Automate and run tests regularly | Detects regressions early[4][6] |
| Avoid over-mocking and unstable selectors | Keeps tests robust and meaningful[8][9][10] |
---
By following these cursor rules, coding agents can ensure their JavaScript and React application tests are robust, maintainable, and focused on delivering real value to users and developers alike.[1][3][2][4][5][6][7][8][9][10]
Sources
[1] www.testingjavascript.com https://www.testingjavascript.com
[2] Testing JavaScript | Testing JavaScript https://www.testingjavascript.com/
[3] Testing JavaScript | Testing JavaScript https://www.testingjavascript.com
[4] How to Test JavaScript Applications: Tools, Techniques, and Best ... https://codedamn.com/news/programming/test-javascript-applications-tools-techniques
[5] How to test React Apps - BrowserStack https://www.browserstack.com/guide/how-to-test-react-apps
[6] Testing in JavaScript: Best Practices and Tools - DEV Community https://dev.to/mattryanmtl/testing-in-javascript-best-practices-and-tools-4bkb
[7] Testing React Components: Best Practices and Tools - LinkedIn https://www.linkedin.com/pulse/testing-react-components-best-practices-tools-alex-lomia
[8] Javascript Unit Testing Best Practices to Follow - BrowserStack https://www.browserstack.com/guide/javascript-testing-best-practices
[9] goldbergyoni/javascript-testing-best-practices - GitHub https://github.com/goldbergyoni/javascript-testing-best-practices
[10] 13 JavaScript Testing Best Practices You Should Know [2025] https://www.lambdatest.com/blog/javascript-testing-best-practices-you-should-know/
[11] Testing JavaScript Applications | <Blog /> - Colin Brooks https://colinrobertbrooks.github.io/blog/testing-javascript-applications/
[12] How to properly test react applications? What are the best ways to ... https://www.reddit.com/r/reactjs/comments/17kfmuf/how_to_properly_test_react_applications_what_are/
[13] Effective Testing in JavaScript | AppSignal Blog https://blog.appsignal.com/2024/05/08/effective-testing-in-javascript.html
[14] React Functional Testing Best Practices - Daily.dev https://daily.dev/blog/react-functional-testing-best-practices
[15] How to properly learn testing in JavaScript/TypeScript in 2022 - Reddit https://www.reddit.com/r/reactjs/comments/y4qbap/how_to_properly_learn_testing_in/
[16] How to properly unit test a React component? - Stack Overflow https://stackoverflow.com/questions/38815439/how-to-properly-unit-test-a-react-component
[17] JavaScript Unit Testing Tutorial | BrowserStack https://www.browserstack.com/guide/unit-testing-in-javascript
[18] React Testing for Beginners: Start Here! - YouTube https://www.youtube.com/watch?v=8Xwq35cPwYg
[19] Testing Your JavaScript: Best Practices for Robust Applications. https://dev.to/dharamgfx/testing-your-javascript-best-practices-for-robust-applications-428a
[20] Testing React Apps - Jest https://jestjs.io/docs/tutorial-react
[21] JavaScript testing: 9 best practices to learn - LogRocket Blog https://blog.logrocket.com/javascript-testing-best-practices/
[22] Best practices for writing good unit tests for components? : r/reactjs https://www.reddit.com/r/reactjs/comments/wqy6rz/best_practices_for_writing_good_unit_tests_for/
[23] How to quickly test some javascript code? [closed] - Stack Overflow https://stackoverflow.com/questions/8869783/how-to-quickly-test-some-javascript-code