server.js

// CommonJs
/**
 * @type {import('fastify').FastifyInstance} Instance of Fastify
 */
const fastify = require('fastify')({
    logger: true
})

fastify.register(require('@fastify/cors'), (instance) => {
    return (req, callback) => {
        const corsOptions = {
            // This is NOT recommended for production as it enables reflection exploits
            origin: true
        };

        // do not include CORS headers for requests from localhost
        if (/^localhost$/m.test(req.headers.origin)) {
            corsOptions.origin = false
        }

        // callback expects two parameters: error and options
        callback(null, corsOptions)
    }
})

fastify.register(async function (fastify) {
    fastify.get('/', (req, reply) => {
        reply.send({ hello: 'world' })
    })
})

fastify.listen({ port: 3000 }, function (err, address) {
    if (err) {
        fastify.log.error(err)
        process.exit(1)
    }
    // Server is now listening on ${address}
})