Most of the times the default Next server will be enough but sometimes you want to run your own server to customize routes or other kind of the app behavior. Next provides a Custom server and routing so you can customize as much as you want.
Because the Next.js server is just a node.js module you can combine it with any other part of the node.js ecosystem. in this case we are using Fastify to build a custom router on top of Next.
The example shows a server that serves the component living in pages/a.js when the route /b is requested and pages/b.js when the route /a is accessed. This is obviously a non-standard routing strategy. You can see how this custom routing is being made inside server.js.
Preview the example live on StackBlitz:
Execute create-next-app with npm or Yarn to bootstrap the example:
npx create-next-app --example custom-server-fastify custom-server-fastify-app
# or
yarn create next-app --example custom-server-fastify custom-server-fastify-app