How to write effective and clean unit tests in JavaScript

In this article, I'd like to share some best practices for writing unit tests in JavaScript that I've found useful, focusing on clarity, readability, and maintainability.

Table of Contents

• Use it() instead of test()
• Focus on behavior, not implementation
• Use describe() for context
• Split complex tests into multiple ones
• Use the arrange-act-assert pattern
• Use only one action per test
• Avoid overriding global objects
• Avoid mocking internal functions
• Don't break encapsulation for tests
• Handle optional parameters carefully
• Keep expected results close to assertions
• Use meaningful test data
• Group related tests with nested describe blocks
• Use test data builders
• Clean up after tests
• Test edge cases explicitly
• Avoid test interdependence
• Test async code properly
• Use parameterized tests for multiple cases

Use it() instead of test()

Don't:

Do:

Why:

• The it() function creates human-readable phrases that express the intended behavior.
• Improves readability by forming sentences like "it should calculate the cart total with discounts."
• Encourages writing behavior-focused tests rather than technical implementations.

Focus on behavior, not implementation

Don't:

Do:

Why:

• Describing tests in terms of behavior provides a better understanding of expected outcomes.
• Behavior-focused tests are easier to read and serve as effective documentation.
• Helps avoid coupling tests to specific implementations, making them more resilient to changes.

Use describe() for context

Don't:

Do:

Why:

• describe() blocks provide context and group related tests together.
• Starting contexts with "When" enhances clarity about the test scenario.
• Allows for shared setup and teardown with beforeEach and afterEach within the context.

Split complex tests into multiple ones

Don't:

Do:

Why:

• Splitting tests improves readability and maintainability.
• Each test focuses on a single behavior, making it easier to pinpoint issues.
• Facilitates better grouping and organization of tests.

Use the arrange-act-assert pattern

Don't:

Do:

Why:

• The Arrange-Act-Assert (AAA) pattern structures tests clearly, separating preparation, execution, and verification.
• Enhances readability and helps quickly identify issues in failing tests.
• Provides a consistent format that makes tests easier to understand.

Use only one action per test

Don't:

Do:

Why:

• Focusing on a single action in the Act section clarifies what triggers the behavior.
• Multiple actions can create confusion about which action is under test.
• Simplifies debugging and understanding of the test's purpose.

Avoid overriding global objects

Don't:

Do:

Why:

• Directly modifying global objects can lead to side effects that affect other tests.
• jest.spyOn() allows safe mocking and restoration of global methods.
• Prevents hard-to-detect bugs and ensures test isolation.

Avoid mocking internal functions

Don't:

Do:

Why:

• Mocking internal functions couples tests to implementation details, hindering refactoring.
• Tests should verify the output of public functions based on given inputs.
• Promotes better software design and maintainability.

Don't break encapsulation for tests

Don't:

Do:

Why:

• Exporting private functions solely for testing breaks encapsulation and can lead to brittle tests.
• Testing through public interfaces ensures that tests remain valid despite internal changes.
• Enhances code maintainability by allowing internal refactoring without breaking tests.

Handle optional parameters carefully

Don't:

Do:

Why:

• Testing each execution path separately improves clarity.
• Helps ensure all scenarios with optional parameters are adequately tested.
• Makes it easier to identify which case fails if a test does not pass.

Keep expected results close to assertions

Don't:

Do:

Why:

• Placing expected values near assertions improves readability.
• Reduces scrolling and context-switching while reading tests.
• Enhances the flow of the test from setup to assertion.

Use meaningful test data

Don't:

Do:

Why:

• Using realistic test data makes tests more understandable and maintainable.
• Helps identify edge cases and potential issues with real-world data.
• Makes it easier for other developers to understand the purpose of the test.

Group related tests with nested describe blocks

Don't:

Do:

Why:

• Nested describes create a clear hierarchy of test scenarios.
• Makes it easier to understand the context of each test.
• Helps organize tests logically and improves test suite navigation.

Use test data builders

Don't:

Do:

Why:

• Makes test data creation flexible and maintainable.
• Reduces duplication in test setup.
• Makes it easy to create variations of test data while maintaining defaults.

Clean up after tests

Don't:

Do:

Why:

• Ensures each test runs in isolation.
• Prevents test pollution and interdependence.
• Makes tests more reliable and predictable.

Test edge cases explicitly

Don't:

Do:

Why:

• Ensures the code handles all possible scenarios correctly.
• Helps prevent bugs in edge cases.
• Serves as documentation for expected behavior in special cases.

Avoid test interdependence

Don't:

Do:

Why:

• Makes tests more reliable and predictable.
• Easier to debug when tests fail.
• Allows tests to be run in any order.

Test async code properly

Don't:

Do:

Why:

• Ensures asynchronous operations are properly tested.
• Prevents false positives from unhandled promises.
• Makes async test code more readable and maintainable.

Use parameterized tests for multiple cases

Don't:

Do:

Why:

• Reduces code duplication.
• Makes it easy to add new test cases.
• Provides clear documentation of all test scenarios.
• Makes patterns in test cases more visible.

Conclusion

Writing effective and clean unit tests is essential for developing reliable frontend applications. By following these best practices, you can create tests that are easy to understand, maintain, and scale. Focus on testing behaviors rather than implementations, structure your tests clearly with the Arrange-Act-Assert pattern, and ensure each test is isolated and focused.

These practices will help you write more maintainable, reliable, and effective unit tests. The goal is to create tests that are both thorough and easy to understand, helping to catch bugs early while serving as documentation for your code's behavior.