Authentication made easy: Building a secure e-commerce shopping cart with Firebase

Firebase Authentication and Cloud Firestore can support e-commerce carts without requiring visitors to register before shopping by assigning each visitor an anonymous, cryptographically verified Firebase identity and storing cart items under UID-scoped Firestore paths. Security Rules restrict users to their own carts, permit only validated quantity fields, prevent client-side price manipulation, and require order-history queries to explicitly filter by the authenticated owner because rules do not filter query results. When guests create new accounts, credentials can be linked while retaining the same UID and cart, while returning users require deterministic cart reconciliation into their existing account with bounded quantities. Confirmed orders should be created only by a trusted Cloud Run backend or Firebase Callable Function, which verifies Firebase ID tokens, obtains canonical product prices, calculates totals server-side, writes immutable order records with administrative privileges, and clears carts after checkout.

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