browser-routes.js

function browser_routes({
  app,
  auth_config,
  app_type,
  store,
  options,
  middleware,
}) {
  app.post(options.auth_url, middleware, (req, res) => {
    res.send('foo');
  })
}

module.exports = browser_routes;

index.js

const configure_browser_routes = require('./browser-routes');
const configure_server_routes = require('./server-routes');
const some_middleware = require('./some-middleware');

function my_plugin(app, options) {
  const { token_store: store } = options;

  const auth_config = {};

  const app_type = app?.use ? 'express' : 'fastify';

  const middleware = some_middleware(options); 

  configure_browser_routes({
    app,
    auth_config,
    app_type,
    store,
    options,
    middleware,
  });

  configure_server_routes({
    app,
    auth_config,
    app_type,
    store,
    options,
    middleware,
  });
}

module.exports = my_plugin;

server-routes.js

function server_routes({
  app,
  auth_config,
  app_type,
  store,
  options,
  middleware,
}) {
  app.post(options.some_url, middleware, (req, res) => {
    res.send('foo');
  })
}

module.exports = server_routes;

some-middleware.js

function some_middleware(options) {
  return (req, res, next) => {
    if (options.ips.includes(req.ip)) {
      throw new Error('Foo');
    }

    next();
  }
}

module.exports = some_middleware;