Preview deployments: every commit gets its own real, working URL

Every push to any branch (and every pull request) gets its own, fully working, publicly-reachable deployment — not a mockup, not a static preview image, a genuinely running instance of the app with its own URL, isolated from production and from every other branch's preview. This happens with zero configuration beyond connecting the repository in the first place.

CI/CD's own testing-gates lesson covered automated tests running against code — a preview deployment is a complementary, different kind of check: a human (a designer, a product manager, a non-technical reviewer) can click a real link and interact with the actual feature, not read a diff and imagine how it behaves. This closes a real gap automated tests don't: "does this look and feel right" is a question only a real, running instance can answer, and preview deployments make getting one effortless for every single change, not just ones a team remembers to deploy manually to a staging environment.

Vercel's preview deployments support an overlay toolbar letting anyone leave a comment pinned to a specific point on the actual rendered page — feedback tied directly to "this button, right here" rather than a separate, disconnected conversation in a PR description or a chat message describing a location in words. This closes the loop CI/CD's own environments lesson left open: a staging environment a team configures manually still requires someone to separately communicate where on the page an issue is; a pinned comment on a live preview doesn't.

CI/CD & Deployment Pipelines' own domain covered environments and approvals as something a team deliberately sets up — a staging environment, gated behind a manual approval, with its own secrets and configuration. Vercel's preview deployments are a different, narrower thing: automatic, ephemeral, one per branch/PR, torn down (or simply superseded) as development moves on, with no manual approval step required to create one at all. A real project often uses both: Vercel's automatic previews for rapid, per-commit feedback, and a more deliberate staging environment (possibly on Vercel too, via a dedicated branch) for a final gate before production.

Preview deployments can read different environment variable values than production (lesson 6 covers this precisely) — a preview pointing at a test database or a sandboxed third-party API key, while production points at the real ones, configured once in the dashboard and applied automatically based on which kind of deployment is currently running, with no manual switching required per branch.