protect()

The protect() function returns Express middleware that blocks unauthenticated requests.

Signature

protect(
  onUnauthenticated?: (res: express.Response) => void,
): (req: express.Request, res: express.Response, next: express.NextFunction) => Promise<void>

Import

const {protect} = require('@thunderid/express');

Prerequisites

• Mount thunderID() before this middleware
• Mount cookie-parser before this middleware so the SDK can read req.cookies

Usage

Default 401 Response

index.js

const {protect} = require('@thunderid/express');

app.get('/protected', protect(), (_req, res) => {
  res.send('Protected content');
});

Custom unauthenticated behavior

index.js

const {protect} = require('@thunderid/express');

app.get(
  '/protected',
  protect((res) => res.redirect('/login')),
  (_req, res) => {
    res.send('Protected content');
  },
);

Runtime Behavior

For each request, the middleware:

1. reads req.thunderIDAuth
2. reads the session ID from req.cookies[SESSION_COOKIE_NAME]
3. calls client.isSignedIn(sessionId)
4. either continues to the next handler or rejects the request

Default Behavior

If no onUnauthenticated callback is provided, the middleware returns 401 with an empty response body.

Failure Behavior

• If the SDK client is missing or the session cookie is missing, the middleware rejects the request
• If the session ID is invalid, the middleware rejects the request
• In both cases, it uses your onUnauthenticated callback when provided, or the default 401 response otherwise

Notes

• protect() checks authentication state for the session cookie value, not for the presence of user data on the request
• A common pattern is to redirect to /login for browser routes and use the default 401 response for API routes

Related APIs

• thunderID()
• handleSignIn()
• Redirect Flow