next.js/test/integration/production-config/pages/index.js

import React, {Component} from 'react'

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

    this.state = {
      mounted: false
    }
  }

  componentDidMount () {
    this.setState({mounted: true})
  }

  render () {
    return <p id='mounted'>ComponentDidMount {this.state.mounted ? 'executed on client' : 'not executed'}.</p>
  }
}