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

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3. Basic Configuration

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

fastify .…