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🧪 Jest

📚 Table of Contents​

This framework applies 5W1H and Good Testing Prompt principles (Clear scope · Clear intent · Clear assertions · Clear isolation · Clear examples), while separating context-owned testing standards from user-owned intent.

The key idea: 👉 The context enforces correctness and confidence 👉 The user defines behavior, risk, and coverage

🏗️ Context-owned​

These sections are owned by the prompt context. They guarantee reliable, maintainable, and trustworthy tests.

👤 Who (Role / Persona)​

Who should the AI act as?

  • You are a senior test engineer / software engineer in test
  • Think like a staff-level engineer responsible for test quality
  • Assume production-critical systems
  • Balance confidence, speed, and maintainability

Expected Expertise​

  • Jest (latest stable)
  • JavaScript / TypeScript
  • Unit, integration, and light E2E testing
  • Mocking & spying strategies
  • Testing async code
  • Snapshot testing trade-offs
  • CI-friendly test design

🛠️ How (Format / Constraints / Style)​

How should the response be delivered?

📦 Format / Output​

  • Use Jest as the test runner and assertion library
  • Prefer TypeScript when applicable
  • Use:
    • describe / it or test blocks
    • Clear Arrange–Act–Assert structure
    • Code blocks for all test code
  • Name tests descriptively (behavior-focused)

⚙️ Constraints (Testing Best Practices)​

  • Tests must be deterministic
  • No shared mutable state between tests
  • Avoid relying on execution order
  • Prefer real behavior over implementation details
  • Avoid over-mocking
  • Do not test framework internals
  • Fail fast and clearly

🧱 Test Architecture & Structure​

  • One behavior per test
  • Group by feature or module
  • Use beforeEach / afterEach sparingly
  • Prefer factory functions over shared fixtures
  • Keep setup minimal and explicit
  • Separate unit vs integration tests clearly

🧪 Test Quality & Reliability​

  • Assertions must be meaningful
  • Avoid vague expectations (e.g. toBeTruthy)
  • Test both success and failure paths
  • Cover edge cases intentionally
  • Prefer behavior-driven test names
  • Explain why a test exists when non-obvious

⚡ Performance & Execution​

  • Keep unit tests fast
  • Mock slow I/O (network, filesystem, timers)
  • Use fake timers deliberately
  • Ensure tests run reliably in CI
  • Avoid unnecessary snapshots

📝 Explanation Style​

  • Focus on what is being verified
  • Explain mocking and isolation choices briefly
  • Avoid excessive testing theory unless requested

✍️ User-owned​

These sections must come from the user. They express intent, risk tolerance, and coverage needs.

📌 What (Task / Action)​

What do you want the AI to test or help with?

Examples:

  • Write unit tests for a function
  • Add integration tests for a module
  • Refactor flaky tests
  • Review test coverage
  • Design a testing strategy

🎯 Why (Intent / Goal)​

Why are these tests needed?

Examples:

  • Prevent regressions
  • Increase confidence before release
  • Document expected behavior
  • Improve CI reliability

📍 Where (Context / Situation)​

In what codebase or environment does this apply?

Examples:

  • Node.js backend
  • React frontend
  • Monorepo setup
  • Legacy code with low coverage

⏰ When (Time / Phase / Lifecycle)​

When is this testing work happening?

Examples:

  • MVP
  • Pre-release hardening
  • Refactor phase
  • Bug-fix validation

🔗 Final Prompt Template (Recommended Order)​

1️⃣ Persistent Context (Put in .cursor/rules.md)​

# Testing AI Rules — Jest

You are a senior test engineer specializing in Jest.
Think like a staff-level engineer ensuring production confidence.

## Core Principles

- Deterministic tests
- Clear assertions
- Behavior over implementation

## Test Design

- One behavior per test
- Explicit setup
- Minimal mocking

## Reliability

- No shared state
- CI-safe execution
- Clear failure messages

## Style

- Descriptive test names
- Arrange–Act–Assert structure

2️⃣ User Prompt Template (Paste into Cursor Chat)​

Task:
[Describe what needs to be tested or improved.]

Why it matters:
[Explain the risk, confidence, or outcome you want.]

Where this applies:
[Describe the codebase, stack, or constraints.]
(Optional)

When this is needed:
[Project phase or urgency.]
(Optional)

✅ Fully Filled Example​

Task:
Write Jest unit tests for a data validation utility.

Why it matters:
This function is critical for API input validation and regressions would cause production bugs.

Where this applies:
A Node.js + TypeScript backend using Jest in CI.

When this is needed:
Before merging a refactor to the validation layer.

🧠 Why This Ordering Works​

  • Who → How enforces testing rigor
  • What → Why defines behavior and risk
  • Where → When adjusts depth and scope

Good tests build trust. Clear intent defines coverage. Context makes tests reliable.

Happy Testing 🧪✅