How to Fix CORS Errors in Node.js Fastify

Cross‑Origin Resource Sharing (CORS) errors are one of the most common roadblocks when building APIs with Fastify . This guide walks you through the root causes, provides a solid Fastify‑compatible solution, and shows you how to test it locally and in production.

Why CORS Errors Occur

When a browser requests a resource from a different origin (scheme, host, or port) than the one serving the page, the server must explicitly allow that request via CORS headers . If the headers are missing or malformed, the browser blocks the response and logs an error such as:

Access to fetch at 'https://api.example.com/data' from origin 'http://localhost:3000' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.

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Fastify does not add CORS headers automatically; you have to enable them via a plugin.

Step‑by‑Step Fix Using fastify-cors

1️⃣ Install the official plugin

npm install…