Example counter react component

Counter.js

class Counter extends React.Component {
  state = {
    count: 0
  }

  onClick = () => {
    this.setState(prevState => ({ ...prevState, count: prevState.count + 1 }))
  }

  render = () => (
    <div>
      <button type="button" onClick={this.onClick}>
        Increment
      </button>
      <span>Clicked: {this.state.count}</span>
    </div>
  )
}