This comprehensive guide details the implementation of the OAuth Authorization Code Grant in a React application using FusionAuth as the Identity Provider (IdP), emphasizing its security benefits for single-page applications. The process involves setting up a React app with login, logout, and user data management functionalities, integrated with a Node.js backend using Express. The guide walks through configuring FusionAuth, setting up a directory structure, and coding both client and server components to handle OAuth flows securely. It covers key topics such as server-side session management, exchanging authorization codes for access tokens, introspection for user data retrieval, and updating user registration data. The article provides a step-by-step approach to managing user authentication and data in a secure and scalable manner, while also suggesting further enhancements like user registration, customization of login forms, implementing two-factor authentication, and third-party logins.