> Source: https://kaidn.io/docs/quickstart/preact > Full documentation index: https://kaidn.io/llms.txt Docs menuAll pages, quickstarts and this page’s contentsDocs - [Introduction](https://kaidn.io/docs) - [Quickstarts](https://kaidn.io/docs/quickstart) - [Core concepts](https://kaidn.io/docs/concepts) - [API reference](https://kaidn.io/docs/api) - [Guides](https://kaidn.io/docs/guides) - [Keys & dashboard](https://kaidn.io/docs/keys) - [Glossary](https://kaidn.io/glossary) Web - [JavaScript](https://kaidn.io/docs/quickstart/javascript) - [React](https://kaidn.io/docs/quickstart/react) - [Next.js](https://kaidn.io/docs/quickstart/nextjs) - [Preact](https://kaidn.io/docs/quickstart/preact) - [Vue](https://kaidn.io/docs/quickstart/vue) - [Nuxt](https://kaidn.io/docs/quickstart/nuxt) - [Angular](https://kaidn.io/docs/quickstart/angular) - [Svelte](https://kaidn.io/docs/quickstart/svelte) Mobile - Androidsoon - iOSsoon - React Nativesoon - Fluttersoon Server - [Node.js](https://kaidn.io/docs#quickstart) - [PHP](https://kaidn.io/docs/quickstart/php) - C#/.NETsoon - Gosoon - Javasoon - [Python](https://kaidn.io/docs/quickstart/python) On this page - [1. Get your publishable key](#step-1) - [2. Set up your project](#step-2) - [3. Build the signup form](#step-3) - [4. Add the provider](#step-4) - [5. Collect on submit](#step-5) - [6. Score it on your server](#step-6) - [7. Test it](#step-7) - [The other hook](#more) - [Next steps](#next) # Preact quickstart Add Kaidn to a Preact app and give every visitor a stable device id you can send to your backend. Scaffolded with Vite, and the example is the one most people start with: stopping the same person opening account after account. `@kaidn/preact` imports `preact` and `preact/hooks` directly, so nothing here needs a `preact/compat` alias. Estimated time: under 10 minutes ### Before you start - **Node 20+ and npm**, to run Vite. - **Preact 10 or later.** Astro islands and Fresh both work. - **A free Kaidn account.** [Register](https://kaidn.io/register): 10,000 events a month, no card. **This is the frontend half, and on its own it blocks nothing.** By the end you will have a device id. That only becomes fraud prevention when your server sends it to `/v1/score` and acts on the verdict, so finish with a backend quickstart: [Python](https://kaidn.io/docs/quickstart/python), [PHP](https://kaidn.io/docs/quickstart/php) or [Node.js](https://kaidn.io/docs#quickstart). ## 1. Get your publishable key - [Create an account](https://kaidn.io/register) if you do not have one. - Go to **Fraud Scoring API → Device trackers**, create a tracker, and list the domains it may run on. Include `localhost` while you build. - Copy the **publishable key**. It starts `pk_live_`. **This key is meant to be visible in your bundle.** It is domain-locked and can only send fingerprints: it cannot score, read your data, or work from a site you did not authorise. Your `kdn_live_` secret key is the opposite, and passing one to the provider throws immediately rather than letting it ship to every visitor. ## 2. Set up your project Skip to step 3 if you have a project already. Terminal ``` npm create vite@latest kaidn-preact-quickstart -- --template preact cd kaidn-preact-quickstart npm install ``` Run it and open `http://localhost:5173`, Vite's default. Terminal ``` npm run dev ``` ## 3. Build the signup form A component to attach to. Create `src/SignupForm.jsx`: src/SignupForm.jsx ``` export default function SignupForm({ onSubmit, isLoading }) { return (
{ e.preventDefault(); onSubmit(new FormData(e.currentTarget)); }} >

Create an account

); } ``` Preact accepts `class` and `for` as well as `className` and `htmlFor`. Either is fine; the plain HTML names are used here because that is what Preact's own templates ship. And enough CSS to see it. Append to `src/style.css`: src/style.css ``` .wrap { max-width: 380px; min-height: 100vh; margin: 0 auto; display: flex; flex-direction: column; justify-content: center; gap: .5rem; padding: 1rem; text-align: left; } input { padding: .6rem; border: 1px solid #ccc; border-radius: 6px; font: inherit; } label { font-size: .85rem; color: #555; } button { margin-top: .75rem; padding: .7rem 1.2rem; font: inherit; cursor: pointer; background: #111; color: #fff; border: 0; border-radius: 6px; } button:disabled { opacity: .6; cursor: not-allowed; } ``` ## 4. Add the provider Terminal ``` npm install @kaidn/preact ``` Wrap your app once, at the root. The provider holds configuration and **collects nothing on its own**, so it is safe on every page including the ones with no signup on them. src/main.jsx ``` import { render } from "preact"; import { KaidnProvider } from "@kaidn/preact"; import { App } from "./app.jsx"; import "./style.css"; render( , document.getElementById("app") ); ``` **For production, use an environment variable.** Vite exposes anything prefixed `VITE_`, so `import.meta.env.VITE_KAIDN_PK` keeps it out of your repository. It is not a secret, but a key you can rotate without a code change is worth having. #### Why not @kaidn/react with preact/compat You can. `preact/compat` aliases React imports, and [@kaidn/react](https://kaidn.io/docs/quickstart/react) runs fine underneath it. The catch is that it works only while that alias is in your bundler config, and the failure when someone removes it is a build error in a fraud dependency that nobody expects to be React. This package imports `preact` and `preact/hooks`directly, so there is nothing to alias and nothing anyone's config can break. The API is deliberately identical, so moving between the two is an import change and nothing else. ## 5. Collect on submit `useDeviceId()` collects when you ask it to, not on mount. That puts the work at the moment somebody acts, which is when the signal is freshest, and keeps fingerprinting off pages that do not need it. src/app.jsx ``` import { useDeviceId } from "@kaidn/preact"; import SignupForm from "./SignupForm.jsx"; export function App() { const { getDeviceId, isLoading } = useDeviceId(); async function handleSubmit(form) { // Never throws. A blocked script or an ad blocker returns null, and your // signup carries on with one signal fewer. const deviceId = await getDeviceId(); await fetch("/api/signup", { method: "POST", headers: { "content-type": "application/json" }, body: JSON.stringify({ email: form.get("email"), password: form.get("password"), deviceId, // your server scores THIS }), }); } return ; } ``` The hook gives you four things: - `getDeviceId()` collects and returns `string | null`. Never throws. - `deviceId` is the last id collected, or null before the first call. - `isLoading` is true while a collection is in flight. Disable your button with it. - `error` is whatever went wrong last, for your logging rather than your user. Note what is *not* here: no verdict and no decision. The browser is an untrusted place to make one, so it never sees a score. It produces an id; your server produces the judgement. **A double-clicked button fingerprints once, not twice.** The hook keeps one collection in flight at a time. Preact has no StrictMode double-invoke to work around, so that guard is there purely for the impatient-visitor case, which is the one that actually happens in production. ## 6. Score it on your server This is the step that turns a device id into fraud prevention. Everything before it collects; nothing before it decides. Your `/api/signup` handler takes the id and passes it to `/v1/score`. Node, since you are already in a JavaScript project: C#/.NET*soon*Go*soon*Java*soon* npm install @kaidn/sdk api/signup.js ``` import { Kaidn } from "@kaidn/sdk"; // Your SECRET key. Server only, never the browser. const kaidn = new Kaidn({ apiKey: process.env.KAIDN_API_KEY }); // Express, Fastify, Hono, a Next.js route handler: the call is the same. app.post("/api/signup", async (req, res) => { let r; try { r = await kaidn.score({ event: "signup", ip: req.ip, email: req.body.email, device_id: req.body.kaidn_device_id, // the id the browser collected }); } catch { // FAIL OPEN. An outage in your fraud vendor must never become an // outage in your signup form. return res.json({ ok: true }); } if (r.verdict === "block") { // Do not name the signal. It teaches the next attempt. return res.status(403).json({ error: "We could not create that account." }); } const user = await createUser(req.body); if (r.verdict === "review") await flagForReview(user.id, r.event_id, r.reason_text); res.json({ ok: true }); }); ``` Three answers, and **you** decide what each one means. Kaidn never blocks anybody on your behalf. ## 7. Test it Terminal ``` npm run dev ``` Open `http://localhost:5173`, fill the form, submit, and look at the network tab. Your `/api/signup` request carries the id: Output ``` { "email": "you@example.com", "deviceId": "8f1c2ae9d4b7c3e05a1f6b28d9074e3c" } ``` **Turn ad blockers off on localhost while you test.** Any fingerprinting script can be blocked by an extension, and when that happens you get nothing rather than a cautious answer. We measured what that does to a competitor and [published the numbers](https://kaidn.io/blog/fingerprinting-blocked-agent). That is why the hook returns null instead of throwing, and why the device id is one signal in a score rather than the whole thing. ## The other hook #### useSessionWatch: catch what one fingerprint cannot `useDeviceId`answers "who is this" at one instant. Some of the most useful signals only exist across *time*, because they are a change rather than a value: a VPN that drops mid-session and leaks the real home IP, a session that starts masking partway through, one device seen from a dozen addresses. None of those can be read from a single page load. src/AccountLayout.jsx ``` import { useSessionWatch } from "@kaidn/preact"; export default function AccountLayout({ user, children }) { // Re-beacons the same device about once a minute while the tab is open, // so a connection change becomes visible. Stops on logout and unmount. useSessionWatch({ active: Boolean(user) }); return children; } ``` **It costs nothing.** The `/v1/fp` beacon is free and rate-limited; you are billed per scored decision. Watching a session adds signal without adding events, which is why it belongs on a logged-in layout rather than being rationed. #### Server rendering A fingerprint cannot exist on a server, so `deviceId` is null through `preact-render-to-string` and the first hydration pass, and nothing is collected until you call `getDeviceId`. Both renders produce the same null, so there is no mismatch to work around. That holds for Astro islands and Fresh as well. #### Consent One flag on the provider stops every hook collecting anything: src/main.jsx ``` ``` Fingerprinting reads properties of a visitor's device, which in several jurisdictions needs a lawful basis *before* it happens rather than a note in a policy afterwards. This library will not assume one on your behalf. #### What this package will never do Two hooks is the whole surface, and that is a boundary rather than a gap. The rest of the API, `config`, `lists`, `label`, `forget`, `events`, `stats`, `batch`, is reachable only with your **secret key**, and a secret key in a browser bundle is a leak found by whoever reads the bundle first. They would also be decisions a visitor could edit. Every one of them lives in the server clients: [full reference](https://kaidn.io/docs/api). ## Next steps **Nothing is being scored yet.** Send that `deviceId` to your backend and pass it to `/v1/score`, which is where a verdict and a decision actually happen. [Python backendFastAPI, six steps, the other half of this](https://kaidn.io/docs/quickstart/python)[PHP backendPlain PHP on any host, including shared](https://kaidn.io/docs/quickstart/php)[Node.js backend@kaidn/sdk, the oldest and most complete client](https://kaidn.io/docs#quickstart)[Device identityWhy a fingerprint is not a person, and the three rungs](https://kaidn.io/docs/concepts#identity) Worth reading once the loop is closed: [the device token](https://kaidn.io/docs/concepts#device-token), which upgrades a guessed identity to a remembered one and is the single biggest accuracy win available to you.