如何修复“TypeError: undefined is not an object”？

Question

Here I'm trying to update index when I'm swiping:

export default class App extends PureComponent {
  static propTypes = {
      onIndexChanged: PropTypes.func,
      index: PropTypes.number,
  };

  index = 0;

  constructor(props) {
      super(props);
      const { index } = this.props;

      this.index = index;
  }

  ref = (el) => {
      this.swiper = el;
  };

  onIndexChanged = (newIndex) => {
      const { onIndexChanged } = this.props;
      this.index = newIndex;

      if (onIndexChanged) {
          onIndexChanged(newIndex);
      }
  };

  componentWillReceiveProps({ index }) {
      if (index !== this.index) {
          const difference = index - this.index;
          this.swiper.scrollBy(difference);
      }
  }

  render() {

      return (
          <RNSwiper
              {...this.props}
              ref={el => this.ref(el)}
              onIndexChanged={this.onIndexChanged}
          />
      );
  }
}

But I'm getting this: TypeError: undefined is not an object (evaluating '_this.state.children[_this.state.index]'). How can I fix that?

Answer 1

在从对象中获取一些值之前，请先检查 null 或 undefined，请参考下面的代码，它可能会解决您的问题。

_this.state.children && _this.state.children[_this.state.index]