example 1 for react best practice to handle this

Foo.js

class Foo extends Components {
  constructor(props) {
    super(props);
    this.state = { message: "Hello" };
  }
  logMessage() {
    const { message } = this.state;
    console.log(message);
  }
  render() {
    return (
      <input type="button" value="Log" onClick={this.logMessage.bind(this)} />
    );
  }
}