Can we build our own erpnext frontend using react.js, completely ?
Yes you can. You can use the ERPNext as backend for your React Native App using API.
For more information on using APIs, please read this guide: https://docs.erpnext.com/docs/v12/user/manual/en/setting-up/articles/integrating-erpnext-with-other-application
Thanks, @mangroliya . I want to do that in react.js ( for web) not in react native.