next.js/examples/with-react-helmet/pages/_app.js

import React from 'react'
import App, { Container } from 'next/app'
import Helmet from 'react-helmet'

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

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

    return { pageProps }
  }

  render () {
    const { Component, pageProps } = this.props

    return (
      <Container>
        <Helmet
          htmlAttributes={{ lang: 'en' }}
          title='Hello next.js!'
          meta={[
            { name: 'viewport', content: 'width=device-width, initial-scale=1' },
            { property: 'og:title', content: 'Hello next.js!' }
          ]}
        />
        <Component {...pageProps} />
      </Container>
    )
  }
}