renderPropCounter.js

class Counter extends Component {
  constructor(props) {
    super(props);
    this.state = { count: props.initCounter };
  }

  increase = () => this.setState(prevState => ({ count: prevState.count + 1 }));
  decrease = () => this.setState(prevState => ({ count: prevState.count - 1 }));

  render() {
    const { count } = this.state;
    return (
      <div>
        {this.props.children({
          count,
          increase: this.increase,
          decrease: this.decrease
        })}
      </div>
    );
  }
}