// 1. Check that the user is logged in.

// 2. (On server) generate user registration request and save it in session:
var u2f = require('u2f');

var req = u2f.request("<appId>");
session.authRequest = req;

// 3. Send req to the client and use U2F API to request registration, then send result back.
// (On client)  u2f.register([req], [], function(res) { /* send res back to server */ })

// 4. (Server) Check registration result.
var checkres = u2f.checkRegistration(session.authRequest, res);

if (checkres.successful) {
    // Registration successful, save 
    // checkres.keyHandle and checkres.publicKey to user's account in your db.
} else {
    // checkres.errorMessage will contain error text.
}

// 1. Check that the user is logged in using password authentication (or some other way).

// 2. (On server) generate user sign request using keyHandle from user account.
var req = u2f.request("<appId>", user.keyHandle);
session.authRequest = req;

// 3. (On client) use U2F API to request signature.
// u2f.sign([req], function(res) { /* send res back to server */ });

// 4. (On server) check signature using publicKey from user account.
var checkres = u2f.checkSignature(session.authRequest, res, user.publicKey);

if (checkres.successful) {
    // User is authenticated.
} else {
    // checkres.errorMessage will contain error text.
}