counter.jsx

import React from 'react';
import ReactDOM from 'react-dom';

class Counter extends React.Component {

  constructor(props) {
    super(props);
    this.state = {count: props.initialCount};
  }

  incrementCount() {
    this.setState({ count: this.state.count + 1 });
  }
  
  decrementCount() {
    this.setState({ count: this.state.count - 1 });
  }
  
  render() {
    return (
    <div>
      <div className="my_counter">
        <font size='+2'>Click counter: {this.state.count}</font>
      </div>
      <button type="Increment" id="but_up" onClick={ e => this.incrementCount(e)}>Increment</button>
      <button type="Decrement" id="but_down" onClick={ e => this.decrementCount(e)}>Decrement</button>
    </div>
    )
  }
  
}

Counter.propTypes = { initialCount: React.PropTypes.number };
Counter.defaultProps = { initialCount: 0 };

ReactDOM.render(<Counter initialCount={42}/>, document.getElementById('counter'));