getInitialProps

_app.js

function MyApp({ Component, pageProps, userAgent }) {
  // pass userAgent to Framework7's App component 
  return (
    <App
      url={url}
      routes={routes}
      userAgent={userAgent}
    >
    {/* ... */}
    </App>
  )
}

MyApp.getInitialProps = async ({ctx}) => {
  // get user-agent string on server-side
  if (ctx && ctx.req && ctx.req.headers) {
    // pass it to our component props
    return {
      userAgent: ctx.req.headers['user-agent']
    }
  }
  return {};
}

export default MyApp