Make the button active if the data is changed

How to track changes if the value of the input field is already there. Probably it will be necessary to make the button active

class NameForm extends React.Component {
  constructor(props) {
    super(props);
    this.state = {value: ''};

    this.handleChange = this.handleChange.bind(this);
    this.handleSubmit = this.handleSubmit.bind(this);
  }

  handleChange(event) {
    this.setState({value: event.target.value});
  }

  render() {
    return (
      <form onSubmit={this.handleSubmit}>
        <label>
          Name:
          <input type="text" value="John" />
        </label>
        <input type="submit" value="send" />
      </form>
    );
  }
}