How do I maintain scroll position when loading older messages?
Before loading, save the current scrollHeight. After new messages are prepended, set scrollTop to (new scrollHeight - old scrollHeight). This keeps the user's view position stable instead of jumping to the top when older messages are loaded.
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More FAQs in Chat Message Persistence and History Loading and Paginating Messages
REST endpoint GET /api/chat/:targetUserId?page=1&limit=50. Query Message with $or for both directions, sort by createdAt -1, skip (page-1)*limit, limit. Return messages reversed (oldest first) with pagination metadata (total, totalPages, hasMore). Use compound index on (senderId, receiverId, createdAt) for efficiency.
On scroll to top, load the next page of older messages and prepend to the list. Maintain scroll position by saving the previous scrollHeight and setting scrollTop after new messages are added. Stop loading when hasMore is false.
In the message:send handler, save to MongoDB with Message.create(), update the Conversation model (lastMessage, lastMessageAt, unreadCount), then broadcast to the room via io.to(roomId).emit('message:received', message). Use a callback acknowledgment to confirm success.
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