next.js/packages/next/src/client/next.ts

import './webpack'
import '../lib/require-instrumentation-client'

import { initialize, hydrate, version, router, emitter } from './'

declare global {
  interface Window {
    next: any
  }
}

window.next = {
  version,
  // router is initialized later so it has to be live-binded
  get router() {
    return router
  },
  emitter,
}

initialize({})
  .then(() => hydrate())
  .catch(console.error)