The auth0-spa-js repository is a JavaScript/TypeScript SDK for authenticating Single Page Applications (SPAs) with Auth0. It implements the Authorization Code Flow with PKCE, providing methods for user login (redirect and popup), silent token acquisition, token refresh, logout, and user information retrieval. The SDK includes advanced features such as Multi-Factor Authentication (MFA), passkey/WebAuthn support, DPoP (Demonstrating Proof of Possession) for token security, Multi-Resource Refresh Tokens (MRRT), and custom token exchange per RFC 8693. Architecture-wise, it is modular with separate concerns: core authentication (Auth0Client), caching (memory and localStorage), storage (cookies and session storage), Web Worker for token operations, and specialized modules for MFA, passkeys, and account management. Key entry points include the `createAuth0Client` factory function and the `Auth0Client` class. The library relies on @auth0/auth0-auth-js for low-level Auth0 interactions and browser-tabs-lock for cross-tab synchronization.