What has to leave the browser, and where to put it

Lesson objective

By the end of this lesson, you will be able to sort your app's functions between browser and server using the trust boundary, and to choose a back-end platform (Supabase, Firebase, or server functions with a separate database) that fits your app, your budget and your data.

Where it fits

Decide on the architecture and protect your secrets

What has to leave the browser, on which platform, and where does each key belong?

Lessons in this module

1. What has to leave the browser, and where to put it (this lesson)
2. Environment variables and secrets, without leaks

What you will learn in the course

This lesson is part of the course Add a database, authentication and payments to your app

• Decide what stays in the browser and what must move to the server, and choose a back-end platform that fits your app, your budget and your data.
• Manage environment variables and secrets (public VITE_ variables, secret keys on the server only, .env files kept out of Git, rotation after a leak).
• Have the agent design a data model and its migrations, then check that data is actually stored and linked to the right user.
• Write, have written and test Row Level Security rules that guarantee each user only accesses their own data and that plan limits are enforced in the database.
• Add sign-up, sign-in, sign-out and password reset, and verify the flows, email confirmation and redirect URLs.
• Integrate Stripe Checkout in test mode (product, price, session created on the server, test cards) without exposing the secret key.
• Hold payment status on the server with signed, deduplicated Stripe webhooks, tested with the Stripe CLI or the Dashboard.
• Apply GDPR basics (information, account deletion, processors) and a go-live checklist before taking real payments.

Related courses

• Build and ship a web app with an AI coding agentJunior · ~2 hr 45 min
• Work with Claude Code on a real project: context, planning, reviewAdvanced · ~3 hr
• Git for PMs: ship as a team without putting production at riskAdvanced · ~3 hr 30 min