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Use Case 7: "I inherited legacy code I don't understand"

Scenario: You've been assigned to maintain a 3-year-old module. The original developer left. The code works but is poorly documented and uses unfamiliar patterns.

Time: ~20 minutes

Copilot Features: Ask Mode, Inline Chat, Agent Refactoring

Your Challenge: Understand the code, document it, and refactor safely without breaking functionality.

Step 1: Understand with Ask Mode​

  1. Open a complex file (e.g., api/src/repositories/suppliersRepo.ts)
  2. Select a confusing function
  3. Use Inline Chat (Cmd/Ctrl + I): 
    Explain what this function does, including edge cases and error handling
  4. Close the inline chat
  5. Move to the chat window, clear your history, and provide broader context in Ask Mode: 
    @workspace Explain the repository pattern used in this codebase. 
    How does it handle database connections and error mapping?

Step 2: Add Documentation​

  1. Switch to Agent mode
  2. Prompt: 
    Add comprehensive JSDoc comments to all functions in suppliersRepo.ts.
    Include parameter descriptions, return types, and example usage.
  3. Copilot will add structured documentation. Review the diff and refine or retain the changes.

Step 3: Refactor with Test Guardrails​

  1. First, ensure tests exist: 
    Review suppliersRepo.test.ts. Are there any missing test cases for edge conditions?
  2. If coverage gaps exist: 
    Add tests for error scenarios: database connection failures, invalid IDs, constraint violations.
  3. Now safely refactor: 
    Refactor suppliersRepo.ts for better readability:
    - Extract complex conditionals into named functions
    - Reduce nested callbacks
    - Add type safety where any types are used

    Run tests after each change to ensure no breaking changes.

Step 4: Generate Architecture Documentation​

  1. Prompt: 
    Create a Mermaid diagram showing the data flow from API route → repository → database for the suppliers module.
    Save it in docs/architecture-suppliers.md
  2. Open and review the file that was created
  3. Note you can render the markdown and diagram by right-clicking the filename in the top tab and selecting "Open Preview"

Step 5: Document Database Schema​

  1. Prompt: 
    Analyze the SQL migrations in api/database/migrations/ and create an ERD (Entity Relationship Diagram) in Mermaid format.
    Include all tables, relationships, and cardinality.

    Save to docs/database-schema.md

Step 6: Create Developer Onboarding Guide​

  1. Prompt: 
    Create docs/ONBOARDING.md with:
    - Prerequisites and setup
    - Architecture overview
    - How to run tests
    - How to add a new API endpoint (step-by-step)
    - Common troubleshooting issues
    - Link to all other documentation

Copilot is great at reviewing code and generating documentation. You can also move this work into a separate app session or delegate it to Copilot cloud agent.

What You Learned​

✅ Ask Mode - Understand complex code without reading line-by-line
✅ Inline Chat - Quick explanations without leaving your file
✅ Agent Refactoring - Safe improvements with test guardrails
✅ Documentation Generation - Diagrams and docs from code

Time Investment: 20 minutes
Value: Hours of code reading condensed, safer refactoring, permanent documentation

Next Steps​

Continue to Agent Skills to learn how to customize Copilot with reusable skills.