next.js/test/integration/static/pages/counter.js

import React from 'react'
import Link from 'next/link'

let counter = 0

export default class Counter extends React.Component {
  increaseCounter () {
    counter++
    this.forceUpdate()
  }

  render () {
    return (
      <div id='counter-page'>
        <div>
          <Link href='/'>
            <a id='go-back'>Go Back</a>
          </Link>
        </div>
        <p>Counter: {counter}</p>
        <button
          id='counter-increase'
          onClick={() => this.increaseCounter()}
        >
          Increase
        </button>
      </div>
    )
  }
}