next.js/examples/with-loading/pages/_app.js

import React from 'react'
import App, {Container} from 'next/app'
import Link from 'next/link'
import NProgress from 'nprogress'
import Router from 'next/router'

const linkStyle = {
  margin: '0 10px 0 0'
}

Router.onRouteChangeStart = (url) => {
  console.log(`Loading: ${url}`)
  NProgress.start()
}
Router.onRouteChangeComplete = () => NProgress.done()
Router.onRouteChangeError = () => NProgress.done()

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>
        <div style={{ marginBottom: 20 }}>
          <Link href='/'><a style={linkStyle}>Home</a></Link>
          <Link href='/about'><a style={linkStyle}>About</a></Link>
          <Link href='/forever'><a style={linkStyle}>Forever</a></Link>
          <Link href='/non-existing'><a style={linkStyle}>Non Existing Page</a></Link>
        </div>

        <Component {...pageProps} />
      </Container>
    )
  }
}