Fix CORS Errors in Node.js Fastify – Complete Guide for Developers

Cross‑Origin Resource Sharing (CORS) errors are a common hurdle when building APIs with Node.js and Fastify. This post walks you through why they happen and how to resolve them step‑by‑step.

1. What Triggers a CORS Error?

When a browser requests a resource from a different origin (scheme, host, or port) than the page it originated from, the server must explicitly allow that request via response headers. If the required Access‑control‑Allow‑Origin header is missing or mismatched, the browser blocks the call and logs a CORS error.

2. Install the Fastify CORS Plugin

npm install fastify-cors

3. Basic Configuration

const fastify = require('fastify')();

fastify.register(require('fastify-cors'), {
  // Allow specific origins or use * for all (not recommended for production)
  origin: ['http://localhost:3000', 'https://my‑frontend.com'],
  methods: ['GET', 'POST', 'PUT', 'DELETE'],
  credentials: true
});

fastify.get('/api/data', async (request, reply) => {
  return { message: 'CORS is now configured!' };
});

fastify.listen({ port: 4000 }, err => {
  if (err) throw err;
  console.log('Server listening on http://localhost:4000');
});

Why This Works

• origin tells the browser which domains are permitted.
• credentials: true adds the Access‑control‑Allow‑Credentials header, needed for cookies or HTTP authentication.

4. Common Pitfalls

5. Advanced: Dynamic Origin Handling

fastify.register(require('fastify-cors'), {
  origin: (origin, cb) => {
    const whitelist = ['http://localhost:3000', 'https://my‑frontend.com'];
    if (whitelist.includes(origin) || !origin) {
      cb(null, true);
    } else {
      cb(new Error('Not allowed by CORS'));
    }
  }
});

This function checks the incoming request origin against a whitelist, allowing you to keep the list in a config file or environment variable.

6. Verify the Fix

Use curl or the browser dev tools:

curl -i -H "Origin: http://localhost:3000" http://localhost:4000/api/data

You should see:

HTTP/1.1 200 OK
access-control-allow-origin: http://localhost:3000
access-control-allow-credentials: true

If the headers appear, the CORS issue is resolved.

Quick Resources

• Download the pre‑configured script here – a ready‑to‑run Fastify project with CORS fully set up.
• Get the complete patch tool – apply the fix to existing services in seconds.
• Access the full repository fix – explore the source, contribute, and adapt it to your workflow.

Feel free to drop a comment if you hit any edge cases, and happy coding!