var el = document.createElement("script");
el.async = true;
el.defer = true;
el.src = "https://cdn.jsdelivr.net/npm/[email protected]${version}/dist/oneaccount.min.js";
document.body.appendChild(el);

<script defer async src="https://cdn.jsdelivr.net/npm/[email protected]<version>/dist/oneaccount.min.js"></script>

if (window.oneaccount) {
  initOneaccount();
} else {
  document.addEventListener("oneaccount-loaded", initOneaccount);
}
​
// init the library
function initOneaccount() {
  window.oneaccount.init("your external id", {
    // NOTE: Please check the Library options page for more customisations
    iOSRedirectURL: "/user", // required
    callbackURL: "/oneaccountauth", // required
  });
}

<!-- NOTE: the classes are important for the library to attach the click bindings -->
<button class="oneaccount-button oneaccount-show">Sign in/up using One account</button>

document.addEventListener("oneaccount-authenticated", function(event) {
  // data contains any data you have returned from the backend
  var data = event.detail;
  // your business logic here
});

npm install -S oneaccount-express

const express = require('express');
const { OneAccount } = require('oneaccount-express');
const Redis = require('ioredis');
const app = express();
const redis = new Redis();
​
app.use(express.json());
​
let oneaccount = new OneAccount({
  // If engine is not provided an in-memory engine will be used. 
  // For production it is recommended to provide an engine:
  // for this example we will use redis but any other database could be used
  engine: {
    // for best results the timeout should match the timeout 
		// set in frontend (updateInterval option, default: 3 minutes)
  	set: (k,v) => redis.set(k, v, "EX", 3 * 60),
  	get: (k) => {
  		let v = redis.get(k); redis.del(k); return v;
  	}
  },
})
​
// The route URL is the callback URL you have set when you created One account app.
app.post('/oneaccountauth', oneaccount.auth, (req, res, next) => {
  // NOTE: req.oneaccount is set when a user is authenticated, 
  // so never return code 200 if this object is not present
  if (!req.oneaccount) {
    return res.status(401).json({ error: 'unauthorized' });
  }
  
  // a user is authenticated and you can implement any logic your application 
  // needs. req.oneaccount holds data sent by the user 
  // after successful authentication
  
  // since One account doesn't differentiate between sign up and sign in, 
  // you can use userId to check if the user signed up already on your website or not.
  // the same way you can access any other data you requested from the user:
  const { userId, firstName } = req.oneaccount;
  // any data returned here would be sent to oneaccount-authenticated event on front-end e.g.:
  return res.json({ firstName });
})
​
// OPTIONAL: in addition you can read error messages if any occured
app.use((err, req, res, next) => {
  console.error(err.devMessage);
  if (!err.status) err.status = 500;
  res.status(err.status).send(err.message);
})
​
app.listen(process.env.PORT || 3000);