ES6+ / React Native class properties: The first one is undefined

Question

I'm trying to use the panResonder in my React Native app. I tried doing so using class properties instead of constructor and super(). Here is the code:

export default class Deck extends Component {
  panResponder = PanResonder.create({
    onStartShouldSetPanResponder: () => true,
    onPanResponderMove: (event, gesture) => {},
    onPanResponderRelease: () => {}
  });
  state = { panResponder };

  renderCards = () => this.props.data.map(item => this.props.renderCard(item));

  render() {
    return <View>{this.renderCards()}</View>;
  }
}

Unfortunately React is complaining that panResponder is undefined. How is that? Do I have to use a constructor here?

The following code is the only that works for me:

export default class Deck extends Component {
  constructor(props) {
    super(props);

    const panResponder = PanResonder.create({
      onStartShouldSetPanResponder: () => true,
      onPanResponderMove: (event, gesture) => {},
      onPanResponderRelease: () => {}
    });
    this.state = { panResponder };
  }

  renderCards = () => this.props.data.map(item => this.props.renderCard(item));

  render() {
    return <View>{this.renderCards()}</View>;
  }
}

Answer 1

panResponder property is defined. panResponder variable isn't.

state = { panResponder } refers to panResponder variable. Instead, it should be:

state = { panResponder: this.panResponder };

Do I have to use a constructor here?

There's no need for explicit constructor function here because class fields are parts of it. Function scope allows to define a variable and use object literal shorthand. It doesn't provide real benefits here, but if there's no need for panResponder property, it could be omitted in favour of a variable:

constructor() {
  const panResponder = PanResonder.create({
    onStartShouldSetPanResponder: () => true,
    onPanResponderMove: (event, gesture) => {},
    onPanResponderRelease: () => {}
  });

  this.state = { panResponder };
}