Unit Testing Skill

Write comprehensive unit tests with full coverage: happy paths, edge cases, and error conditions.

Core Principles

1. Test behavior, not implementation - Tests verify what code does, not how
2. One assertion concept per test - Each test validates a single logical behavior
3. Arrange-Act-Assert structure - Setup, execute, verify
4. Descriptive test names - Name describes scenario and expected outcome
5. Independent tests - No test depends on another's state or execution order
6. Fast tests - Mock expensive operations (network, disk, databases) unless integration testing

Workflow

0. MANDATORY: Use context7 Before Writing Tests

CRITICAL: Before writing ANY tests, use context7 to look up current testing framework documentation and best practices.

Always use context7 to verify:

• Testing framework APIs (Jest, Vitest, pytest, etc.)
• Assertion syntax and available matchers
• Mocking/stubbing patterns
• Testing library utilities (React Testing Library, etc.)
• Framework-specific best practices

Process:

1. Identify testing framework and libraries in use
2. Use context7 resolve <framework> for each
3. Query relevant topics: assertions, mocking, async testing, etc.
4. Review docs before writing test code

Example:

context7 resolve vitest
context7 get-library-docs --library vitest --topic "mocking"
context7 resolve @testing-library/react
context7 get-library-docs --library @testing-library/react --topic "async"

Workflow Steps

1. Use context7 for testing framework docs - Look up current API and patterns
2. Analyze the code - Identify public interfaces, dependencies, edge cases, error conditions
3. Determine test scope - Unit tests for functions/classes, integration tests for interactions
4. Check existing structure - Follow project's existing test organization patterns
5. Select appropriate framework - See language-specific references below
6. Write comprehensive tests covering:
   • Happy path (expected inputs produce expected outputs)
   • Edge cases (empty inputs, boundaries, null/None values)
   • Error conditions (invalid inputs, exceptions, failure modes)
   • State changes (side effects, mutations)

context7 Usage by Language/Framework

Before writing tests, use context7 to look up documentation for:

Language/Framework	context7 Libraries to Query
JavaScript/React	react, vitest or jest, @testing-library/react
Python	pytest, relevant libraries being tested
R	testthat, tidyverse
Rust	rust (built-in testing)
Go	go (testing package)
C++	googletest or gtest
Node.js/Express	express, supertest, jest

Common topics to query:

• "mocking", "assertions", "async testing", "setup teardown"
• "matchers", "fixtures", "test coverage", "integration tests"

Language References

Select the appropriate reference based on the code being tested:

Language/Tool	Reference	Framework
Python	python.md	pytest
JavaScript	javascript.md	Jest / Vitest
R	r.md	testthat
Rust	rust.md	built-in #[test]
Go	go.md	testing + testify
C++	cpp.md	Google Test
SQL (PostgreSQL)	sql.md	pgTAP
Bash	bash.md	bats-core
Ansible	ansible.md	Molecule + ansible-lint
Kubernetes/Kustomize	kubernetes.md	kubeconform + conftest
Docker	docker.md	hadolint + container-structure-test

Test Organization

Follow each language's conventions. When a project has existing tests, match their structure.

Mocking Strategy

Mock external dependencies based on context:

• Always mock: Network calls, external APIs, system time
• Consider mocking: Databases (use test DB or mock based on test type), file system
• Rarely mock: Pure functions, data transformations

Coverage Targets

Aim for comprehensive coverage:

• All public functions/methods
• All code branches (if/else, match arms, error handlers)
• Boundary conditions
• Error handling paths

REMINDER: Always Use context7

Before writing any test code:

1. Use context7 to look up the testing framework's current API
2. Verify assertion syntax and available matchers
3. Check mocking/stubbing best practices
4. Review async testing patterns if needed

Never write tests from memory alone - always verify with current documentation using context7.