Unit Testing

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Unit tests are deterministic tests that exercise a discrete unit of the application, such as a function, method, or UI component, in isolation to determine whether it behaves as expected.

– Testing Glossary

When testing the specs of functions, prefer testing public API (methods, interfaces, functions) to private API: the spec of private functions and methods are meant to change easily in the future, and unit-testing them would amount to writing a Change Detector Test, which is an anti-pattern.

The purpose of unit tests are to:

• Verify the functionality of a unit (method, class, function, etc.) in isolation
• Good for testing hi-complexity logic where there may be many permutations (e.g. business logic)
• Keep Cyclomatic Complexity low through good separations of concerns and architecture

Principles

• Unit tests are low-level and focus on discrete units of the application
• All dependencies are typically replaced with test-doubles to remove non-determinism
• Unit tests are fast to execute
• Test Suite is ran after every code change

Recommended Best Practices

• Run a subset of your test suite based on the part of the code your are currently working on
  • Following TDD practices plus the watch functionality of certain testing frameworks is an easy way to achieve this
• Pre-commit hooks to run the test suite before committing code to version control
  • Verification during PR and during the CI build on the HEAD to verify that earlier verification happened and was effective.
• Discourage disabling of static tests (e.g. skipping tests, ignoring warnings, ignoring code on coverage evaluation, etc)
• Write custom rules (lint, formatting, etc) for common code review feedback

Resources

• Unit Testing by Martin Fowler
• xUnit Patterns

Examples

embed:examples/unit-test.java embed:examples/unit-test.js

Recommended Tooling

Tooling recommendations based on Testing Strategy ADR

Platform	Tools
Android	Framework: JUnit5 Assertion: Google Truth
iOS	XCTest
Web	Framework: jest Assertion & Mocking: expect (jest), jest-dom, others as necessary Code Coverage: instanbul/nyc (jest)
Java BE	Framework: TestNG, JUnit5 Code Coverage: sonar (sonarlint) Mocking: Powermock, Mockitoi Assertion: REST Assured, Truth, TestNG/JUnit5
JS/node BE	Framework: jest Assertion & Mocking: expect (jest) - generic, supertest or nock - http server endpoint, apollo - graphql server testing Code Coverage: instanbul/nyc (jest)