The monolithic frontend/core/chatCore.js (1310 lines) has been successfully refactored into modular TypeScript files. This improves maintainability, type safety, and reduces the chance of bugs (like the reasoning persistence issue).

• Database initialization and schema management
• Handles Dexie setup and migrations (v1-v11)
• Includes reset logic for prototype mode
• Exports: initDb, AppState, DexieDatabase

• Session CRUD operations
• Handles loading, saving, creating, and managing chat sessions
• Includes slugifyTitle utility
• Key Functions:
  • loadSessions / loadChats - Load session list
  • loadSession - Load a single session with tree/branch support
  • saveSession - CRITICAL - Saves session with reasoning persistence
  • createNewChat, selectChat, deleteChat, duplicateChat
  • moveChatToFolder, baseChats, chatsInFolder

• Folder management operations
• Key Functions:
  • loadFolders, createFolder, toggleFolderCollapsed
  • startEditingFolder, commitEditingFolder, cancelEditingFolder

• Message operations and persistence
• Key Functions:
  • ensureChatExistsForEdits - Creates session when needed
  • persistMessageAtIndex - Persists individual message
  • deleteMessage, deleteMessagesBelow
  • rerunFromUser, rerunAssistant - Handles branching in sessions mode

• Data import/export functionality
• Key Functions:
  • exportAllData - Exports entire database
  • importDataReplace - Replace all data with import
  • importDataAppend - Append imported data

• Central export file for backwards compatibility
• Provides the same API as the old chatCore.js
• Makes it easy to import from a single location

• Converted from JS to TypeScript
• Handles SSE streaming for chat jobs
• Key Class: JobManager

• Already converted to TypeScript in previous work
• Handles branching operations

• Already converted to TypeScript in previous work
• Handles streaming logic

• Central type definitions for the entire application
• Key Types: Session, Message, Branch, SessionTree, UIMessage, AppState
• Helper Functions: messageToUIMessage, createDefaultSessionTree, getNextMessageId, etc.

• frontend/core/chatCore.js (replaced by modular TypeScript files)
• frontend/core/jobManager.js (converted to .ts)
• frontend/core/branchManager.js (converted to .ts)
• frontend/experimental/streaming.js (converted to .ts)

• frontend/chatterApp.js - Updated imports to use './core/index.ts'
• frontend/dataIO.js - Updated imports to use './core/index.ts'
• frontend/types.ts - Added helper functions
• deno.json - Updated for TypeScript support

1. Type Safety: All core logic now has TypeScript types, catching errors at compile time
2. Modularity: Each file has a single responsibility
3. Maintainability: Easier to find and update specific functionality
4. Bug Prevention: The reasoning persistence bug was directly caused by scattered data handling
5. Better IDE Support: Better autocomplete and refactoring tools
6. Easier Testing: Smaller modules are easier to test in isolation

The refactoring directly addresses the reasoning persistence bug by:

1. Centralizing data type definitions in types.ts
2. Making saveSession in sessions.ts explicitly handle m.reasoning != null ? String(m.reasoning) : undefined
3. Ensuring streaming.ts updates state.messages[idx].reasoning during streaming
4. Having loadSession populate state.messageReasoningMap from session tree

• Module imports work correctly
• No linter errors
• Create a new chat
• Send a message with reasoning
• Refresh the page
• Verify reasoning persists
• Test branching (fork, regen, switch)
• Test message editing
• Test import/export
• Test folder operations

1. Test the application to ensure all functionality works
2. Optionally convert chatterApp.js to TypeScript (1890 lines - large task)
3. Consider converting other utility files to TypeScript as needed