The core data-handling logic has been successfully migrated to TypeScript and refactored into modular files. This directly addresses the reasoning persistence bug and improves overall code quality.

1. frontend/types.ts (392 lines)

   • Centralized all type definitions
   • Added helper functions: messageToUIMessage, createDefaultSessionTree
   • Defines: Session, Message, Branch, SessionTree, UIMessage, AppState, etc.

2. frontend/core/db.ts (NEW - 175 lines)

   • Database initialization and schema
   • Dexie setup with migrations v1-v11
   • Handles prototype reset logic

3. frontend/core/sessions.ts (NEW - 470 lines)

   • Session CRUD operations
   • loadSessions, loadSession, saveSession (critical for reasoning persistence)
   • createNewChat, selectChat, deleteChat, duplicateChat
   • Session tree and branch management

4. frontend/core/folders.ts (NEW - 93 lines)

   • Folder management
   • CRUD operations for folders

5. frontend/core/messages.ts (NEW - 284 lines)

   • Message operations
   • ensureChatExistsForEdits, persistMessageAtIndex
   • deleteMessage, deleteMessagesBelow
   • rerunFromUser, rerunAssistant with branching support

6. frontend/core/import-export.ts (NEW - 337 lines)

   • Data import/export functionality
   • exportAllData, importDataReplace, importDataAppend

7. frontend/core/index.ts (NEW - 54 lines)

   • Central export file for all core modules
   • Provides backwards-compatible API

8. frontend/core/jobManager.ts (172 lines)

   • Converted from .js
   • SSE streaming for chat jobs
   • Proper TypeScript types for job data

9. frontend/core/branchManager.ts (559 lines)

   • Already converted (previous work)
   • Core branching operations

10. frontend/experimental/streaming.ts (432 lines)

    • Already converted (previous work)
    • Streaming logic with reasoning persistence fix

1. deno.json

   • Added TypeScript compiler options
   • Added import alias: "@/types": "./frontend/types.ts"
   • Enabled strict type checking

2. Updated Imports

   • frontend/chatterApp.js - Uses './core/index.ts'
   • frontend/dataIO.js - Uses './core/index.ts'

• ❌ frontend/core/chatCore.js (1310 lines) → Replaced by modular TypeScript files
• ❌ frontend/core/jobManager.js → Converted to jobManager.ts

The bug was caused by inconsistent data handling across multiple files. Now:

• All data types are centralized in types.ts
• saveSession explicitly handles reasoning field with proper null checks
• streaming.ts updates state.messages[idx].reasoning during streaming
• loadSession populates state.messageReasoningMap from session tree

• Compile-time error detection
• Better IDE autocomplete
• Catch null/undefined issues before runtime

• Each file has a single responsibility
• Easier to find and update code
• Smaller, more manageable files

• Clear separation of concerns
• Easier to test individual modules
• Better code organization

frontend/
├── types.ts (392 lines) - Central type definitions
├── core/
│   ├── index.ts (54 lines) - Central export
│   ├── db.ts (175 lines) - Database init
│   ├── sessions.ts (470 lines) - Session operations
│   ├── folders.ts (93 lines) - Folder management
│   ├── messages.ts (284 lines) - Message operations
│   ├── import-export.ts (337 lines) - Data I/O
│   ├── jobManager.ts (172 lines) - Job streaming
│   └── branchManager.ts (559 lines) - Branching logic
├── experimental/
│   └── streaming.ts (432 lines) - Streaming logic
└── ...

• Database initialization works
• Create new chat
• Send message
• Message appears in UI

• Send message with reasoning enabled
• Verify reasoning appears in UI
• Refresh page
• Verify reasoning still appears
• Switch branches
• Verify reasoning persists across branches

• Fork at user message
• Fork at assistant message (regen)
• Switch between branches
• Delete message with branches

• Edit message
• Delete message
• Delete messages below
• Rerun from user
• Regenerate assistant

• Create folder
• Move chat to folder
• Toggle folder collapsed

• Export all data
• Import data (replace)
• Import data (append)

1. Convert chatterApp.js to TypeScript (1890 lines - large task)

   • Would provide even more type safety
   • Not critical since core logic is already typed

2. Convert other utility files

   • apiClient.js, chatterLib.js, i18n.js, etc.
   • Lower priority

3. Add unit tests

   • Test core functions in isolation
   • Ensure reasoning persistence works correctly

• The migration was done without JSDoc comments to keep files clean and rely on TypeScript's type system
• All imports are backwards-compatible through frontend/core/index.ts
• No breaking changes to the API
• The modular structure makes future refactoring easier