qa-jest-writer

name	qa-jest-writer
description	Generate Jest unit and integration tests for TypeScript/JavaScript projects with jest.mock, snapshots, code coverage, and describe/it patterns.
output_dir	tests/unit

QA Jest Writer

Purpose

Write Jest unit and integration tests from test case specifications (Phase 2 output). Transform structured test cases into executable TypeScript/JavaScript tests using Jest's describe/it patterns, mocking, snapshots, and coverage tooling.

Trigger Phrases

• "Write Jest tests for [module/function]"
• "Generate Jest unit tests from test cases"
• "Create Jest integration tests for [service]"
• "Add Jest tests with mocks for [dependency]"
• "Jest snapshot tests for [component]"
• "Jest test coverage for [file]"
• "Parameterized Jest tests for [function]"
• "Jest async tests for [API handler]"

Test Types

Type	Scope	Focus
Unit	Isolated function/class testing	Single unit, mocked dependencies
Integration	Module interaction, service layer	Real or partial integration, fewer mocks

Workflow

1. Read test cases — From Phase 2 (qa-testcase-from-docs, qa-testcase-from-ui, qa-manual-test-designer)
2. Analyze target code — Identify module under test, dependencies, exports
3. Generate test files — Create {module}.test.ts or {module}.spec.ts
4. Add mocks/fixtures — jest.mock, jest.spyOn, factory functions, test data
5. Verify coverage — Ensure assertions match test case steps, run coverage report

Context7 MCP

Uses Context7 MCP to fetch current Jest documentation when needed. Query for Jest API, matchers, or configuration to ensure generated tests align with latest Jest behavior.

Key Patterns

Pattern	Use Case
describe / it / test	Group and define test cases
beforeEach / afterEach	Setup and teardown per test
jest.mock()	Mock entire modules (hoisted)
jest.spyOn()	Spy on object methods
Snapshot testing	UI components, serializable output
test.each	Parameterized / table-driven tests
async/await, done	Async testing
Custom matchers	Domain-specific assertions

Assertion Patterns

Assertion	Use
toBe	Strict equality (===)
toEqual	Deep equality
toMatchObject	Partial object match
toThrow	Error thrown
toHaveBeenCalledWith	Mock call arguments
resolves / rejects	Promise outcomes

See references/assertions.md for full reference.

Configuration

• jest.config.ts — TypeScript config with moduleNameMapper, transform, coverage
• tsconfig paths — Map aliases (e.g., @/ → src/)
• setupFiles — Global setup (e.g., env vars, polyfills)

See references/config.md for configuration guide.

File Naming

• {module}.test.ts — Preferred (aligns with Jest default)
• {module}.spec.ts — Alternative (common in Angular/Vue)

Place tests next to source (__tests__/ or colocated) or in tests/ per project convention.

Scope

Can do (autonomous):

• Generate Jest unit and integration tests from test case specs
• Add jest.mock, jest.spyOn, manual mocks
• Create snapshot tests for components/output
• Write parameterized tests (test.each)
• Handle async tests (async/await, resolves/rejects)
• Configure jest.config.ts patterns (moduleNameMapper, coverage)
• Use Context7 MCP for current Jest docs

Cannot do (requires confirmation):

• Change production code to make it testable
• Add tests for modules not in test case scope
• Override project Jest configuration without approval

Will not do (out of scope):

• E2E tests (use Playwright/Cypress skills)
• Run tests or interpret coverage reports
• Modify CI/CD pipelines

References

• references/patterns.md — Unit structure, mocking strategies, async, snapshots, error testing
• references/assertions.md — Complete Jest assertion reference by category
• references/config.md — jest.config, tsconfig paths, coverage, setupFiles
• references/best-practices.md — Anti-patterns, AAA, isolation, descriptive names

Quality Checklist

• No hardcoded sensitive data (use fixtures/env)
• Proper mocking (dependencies isolated, no real I/O)
• Assertions specific (no vague toBeTruthy for complex objects)
• Tests independent (order-independent, no shared mutable state)
• Descriptive test names (behavior/outcome, not implementation)
• Each test case step has corresponding assertion(s)
• Async tests use async/await or proper done/resolves

Troubleshooting

Symptom	Likely Cause	Fix
Module not found	Path/alias mismatch	Check moduleNameMapper in jest.config, tsconfig paths
Mock not working	Hoisting / import order	Use jest.mock() at top of file, before imports
Timeout	Async not resolved	Increase jest.setTimeout(), ensure promises resolve
Snapshot drift	Intentional change	Run jest -u to update; verify change is correct
Coverage gaps	Missing branches	Add tests for error paths, edge cases
Spy not called	Wrong target or timing	Verify spy on correct object, call after setup