This is an example Fastify plugin and server from my medium article:

server.js

const fastify = require('fastify')()
const superPlugin = require('./superPlugin.js')

fastify.register(superPlugin, {
  secretCode: 'JavaScript is awesome!'
})

fastify.get('/', (request, reply) => {
  reply.send({ hello: 'world' })
})

fastify.listen(3000, err => {
  fastify.superMethod()
})

superPlugin.js

const fp = require('fastify-plugin')

function superPlugin (fastify, opts, next) {
  fastify.decorate('superMethod', () => {
    console.log(`Secret code: ${opts.secretCode}`)
  })
  next()
}

module.exports = fp(superPlugin, {
  fastify: '>=1.0.0',
  name: 'super-plugin'
})