next.js/test/integration/with-router/pages/_app.js

import App, { Container } from 'next/app'
import React from 'react'
import HeaderNav from '../components/header-nav'

export default class MyApp extends App {
  static async getInitialProps ({ Component, router, ctx }) {
    let pageProps = {}

    if (Component.getInitialProps) {
      pageProps = await Component.getInitialProps(ctx)
    }

    return {pageProps}
  }

  render () {
    const { Component, pageProps } = this.props
    return (
      <Container>
        <HeaderNav />
        <Component {...pageProps} />
      </Container>
    )
  }
}