next.js/test/integration/required-server-files-ssr-404/pages/_app.js

import App from 'next/app'
import { setConfig } from '../lib/config'

setConfig({ hello: 'world' })

function MyApp({ Component, pageProps }) {
  return <Component {...pageProps} />
}

MyApp.getInitialProps = async (appContext) => {
  // calls page's `getInitialProps` and fills `appProps.pageProps`
  const appProps = await App.getInitialProps(appContext)

  return { ...appProps }
}

export default MyApp