regression-io/refactor

Refactor Large Files

Break down large files into smaller, focused modules while preserving functionality.

When to Use

• File exceeds ~300-500 lines
• File has multiple unrelated responsibilities
• File is hard to navigate or understand
• User asks to refactor or modularize
• Before adding more code to an already large file

Process

Step 1: Analyze the File

1. Identify distinct responsibilities/concerns
2. Map dependencies between functions/classes
3. Find natural boundaries for splitting

Questions to ask:

• What are the main "topics" in this file?
• Which functions call each other?
• What could be tested independently?
• Are there clear layers (API, business logic, data)?

Step 2: Plan the Split

Create a refactoring plan:

Original: src/bigfile.js (800 lines)

Split into:
├── src/bigfile.js (100 lines) - Main entry, re-exports
├── src/bigfile/
│   ├── api.js - API/handler functions
│   ├── utils.js - Helper functions
│   ├── types.js - Type definitions
│   └── constants.js - Constants/config

Guidelines:

• Each new file should have ONE clear purpose
• Keep related code together
• Preserve the public API (re-export from original if needed)
• New files should be <200 lines ideally

Step 3: Execute Incrementally

1. Create new files one at a time
2. Move code, update imports
3. Test after each move
4. Keep the original file as an index/entry point

Step 4: Clean Up

• Remove dead code
• Update imports throughout codebase
• Verify no circular dependencies
• Run tests

Anti-patterns to Avoid

• Don't create files with just 1-2 functions (too granular)
• Don't split by arbitrary line count (split by responsibility)
• Don't create new large files while refactoring
• Don't change functionality while refactoring (separate concerns)

Example

Before:

lib/api.js (600 lines)
  - HTTP handlers
  - Validation logic
  - Database queries
  - Response formatting

After:

lib/api.js (50 lines) - Entry point, re-exports
lib/api/handlers.js - HTTP route handlers
lib/api/validation.js - Input validation
lib/api/queries.js - Database operations
lib/api/responses.js - Response formatting