counter class

Counter.js

class Counter {
  state = {
    value: this.props.defaultValue
  };
  
  handlePlusOne = () => {
    this.setState(() => ({ value: this.state.value + 1 }));
  };
  
  render() {
    return (
      <>
        <div>{this.state.value}</div>
        <button onClick={this.handlePlusOne}>+</button>
      </>
    );
  }
}