Refactoring Typescript

Refactoring TypeScript: Separating Business Logic from System Interactions

When refactoring code to separate concerns, read and follow the policy guide:

Policy: docs/policies/refactoring-typescript.md

When This Applies

• Refactoring files that mix business logic with system calls (file I/O, HTTP, shell, DB)
• Extracting interfaces from tightly coupled code
• Creating *.system.ts wrappers for existing external dependencies
• Adding dependency injection to functions or classes that instantiate SDK clients
• Writing mock implementations for newly extracted interfaces
• Migrating from mock.module() to interface-based DI
• Improving test coverage by making business logic testable

Refactoring Steps (Summary)

1. Identify system boundaries -- find mixed business logic + system calls
2. Define interface -- {domain}-interface.ts with I{Name} contract
3. Create system implementation -- {domain}.system.ts, thin wrapper only
4. Create mock implementation -- tests/mocks/{domain}-mock.ts
5. Refactor business logic -- accept interface param with default value
6. Write tests -- use mock for fast, deterministic tests
7. Configure coverage -- add **/*.system.ts to exclusion

Anti-Patterns to Flag

• Business logic inside *.system.ts files
• Overly broad interfaces combining multiple responsibilities
• Direct dependency on concrete class instead of interface
• Missing interface for external dependency

Read the full guide for code examples, common patterns (file system, shell, HTTP), and migration checklist.

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