flag example.jsx

//////////// x-teaser ////////////

const Teaser = ({ teaser, enableStandfirst }) => <div class="o-teaser">
  <h1>{teaser.title}</h1>
  {enableStandfirst && <h2>{teaser.standfirst}</h2>}
</div>

//////// next-front-page /////////

app.get('/', (req, res) => {
  res.send(
    renderToString(
      <Teaser enableStandfirst={res.locals.flags.get('enableStandfirst') === 'variant'} />
    )
  )
})