Unit Testing

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


  • 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
  • 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



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

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)
Last modified October 13, 2022: Corrected broken links (#8) (d324154)