> ## Documentation Index
> Fetch the complete documentation index at: https://crosslink.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.
# Quickstart
> A desktop app with a secure mobile companion, in about five minutes
You will build a desktop host, a mobile page, and nothing else. Crosslink
provides the pairing screen, the QR, the mobile onboarding, the installable PWA,
the offline screen and the reconnection.
## Prerequisites
* Node.js >= 20.19 (22 recommended)
* npm
## 1. Install
```bash theme={null}
mkdir my-crosslink-app && cd my-crosslink-app
npm init -y
npm install @crosslink/sdk-node
```
The browser SDK is served to your pages by the host, so there is no bundler and
no second install.
## 2. Write the host
`host.mjs`:
```js theme={null}
import { createCrosslinkServer } from "@crosslink/sdk-node";
const notes = [{ id: "1", title: "Hello from your computer" }];
const host = createCrosslinkServer({
application: {
id: "com.example.notes",
name: "Example Notes",
shortName: "Notes",
accentColor: "#f97316",
backgroundColor: "#101014",
appearance: "dark"
},
// Your mobile UI. Crosslink serves it, plus the manifest, service worker,
// icons, browser SDK and all of its own onboarding screens.
mobile: { entry: "./mobile/index.html" },
capabilities: [
{ id: "notes.read", title: "Read your notes", risk: "low" },
{ id: "notes.write", title: "Create notes", risk: "medium" }
],
lan: { enabled: true, bind: "all" }
});
host.expose("notes.list", () => notes, { capability: "notes.read" });
host.expose(
"notes.create",
({ title }) => {
const note = { id: String(notes.length + 1), title };
notes.push(note);
return note;
},
{ capability: "notes.write" }
);
await host.start();
console.log(host.describeMobileDelivery().message);
```
## 3. Write the desktop page
`public/index.html`:
```html theme={null}
Example Notes
Example Notes
```
Serve it from a loopback-bound server with Crosslink's control surface mounted:
```js theme={null}
import http from "node:http";
import { readFile } from "node:fs/promises";
const crosslink = host.createControlHandler({
fallback: async (req, res) => {
res.writeHead(200, { "content-type": "text/html" });
res.end(await readFile("./public/index.html"));
}
});
http.createServer((req, res) => void crosslink(req, res)).listen(8100, "127.0.0.1");
```
The control surface only answers requests from this machine — it mints pairing
codes and revokes devices, so it refuses anything else.
## 4. Write the mobile page
`mobile/index.html`:
```html theme={null}
Notes
```
There is no manifest link, no service-worker registration, no SDK script tag and
no pairing screen here. Crosslink injects what the page needs and has already
run pairing, install and reconnection by the time `onConnected` fires.
## 5. Run it
```bash theme={null}
node host.mjs
```
Open `http://127.0.0.1:8100`, scan the QR with a phone on the same Wi-Fi, and
Crosslink takes it from there: pairing and then your notes list. The phone loads
the desktop's plain-HTTP LAN address in this development mode, so the browser
does not register a Service Worker or provide the durable installed-PWA
lifecycle. The secure published mode linked under **Before you ship** is the
Add to Home Screen path.
## What you did not write
* a pairing screen, QR generation, or a nine-digit code UI
* a pairing-session fetch, refresh timer or expiry handler
* a manifest, a service worker, or icon generation
* Add to Home Screen or Continue in Browser
* an offline screen, a reconnect loop, or revocation handling
* a bundler configuration
## Before you ship
Read [Durable Origins](/client/durable-origin). Add to Home Screen and the
cached offline screen depend on the origin the phone loaded, and a plain-HTTP
LAN address gives you neither. Your host says which case you are in:
```ts theme={null}
host.describeMobileDelivery().message;
```
## Next steps
The desktop pairing UI and its options
What happens between the scan and your app
Making an installed app survive the desktop going offline
Capability-gated methods and events