next.js/test/fixtures/basic/pages/stateful.js

import React, { Component } from 'react'

export default class Statefull extends Component {
  constructor (props) {
    super(props)

    this.state = { answer: null }
  }

  componentWillMount () {
    this.setState({ answer: 42 })
  }

  render () {
    return <div>
      <p>The answer is {this.state.answer}</p>
    </div>
  }
}