WA Reader is a simple web-app to view your backed-up WhatsApp chats in a WhatsApp Web user-interface. You upload the ".txt" file of your chat which you can get from the Email Conversation button in your WhatsApp.
It's free and code is open sourced and you can find the source code here.
Note: WA Reader was built to read a piece of my personal chat, so no guarantees on it working all the time.
It happens since each mobile device and WhatsApp version has it's own way of storing the chats. You can open a new issue on the GitHub repository and share a small piece of chat (if possible) so that the viewer can be updated according to the formatting of your device and WhatsApp version.
Do mention your WhatsApp version, mobile OS and device name for better tracking.
Your WhatsApp chats are automatically backed up and saved daily to your phone's memory. Depending on your settings, you can also periodically backup your WhatsApp chats to Google Drive. If you uninstall WhatsApp from your phone, but don't want to lose any of your messages, be sure to manually back up your chats before uninstalling. To backup your chats, go to WhatsApp > Menu Button > Settings > Chats > Chat backup > Back Up.
Read More: https://faq.whatsapp.com/en/android/23756533
Your WhatsApp messages are automatically backed up and saved daily to your phone's memory. Depending on your settings, you can also periodically back up your WhatsApp chats to OneDrive. If you uninstall WhatsApp from your phone, but don't want to lose any of your messages, be sure to manually back up or export your chats before uninstalling.
Read More: https://faq.whatsapp.com/en/wp/22548236