Question

我目前正在尝试在其中一种视图中实现React / Redux并遇到异常：

TypeError：无法读取未定义的属性“ newsItems”

在mapStateToProps行的newsItems: state.newsItems,函数中发生错误

我的reducer将newsItems初始化为一个空数组，所以我不确定为什么会是undefined

我的组件如下所示：

class PublicLayout extends Component {
  displayName = PublicLayout.name

  constructor(props) {
    super(props);
  }

  retrieveNewsItemsAndArticles = (newsType) => {
    //this method populates the articles and newsitems in the store.
  };

  updateNewsItems = (newsType) => {
    this.retrieveNewsItemsAndArticles(newsType);
  }

  render() {
      if(this.props.articles == null || this.props.newsItems == null){
        this.retrieveNewsItemsAndArticles(0);
      }
      else{
          if(initialLoad){
              initialLoad = false;
          }
      }

    let contents = this.props.articles == null || this.props.newsItems == null ? (
      <p>
          <em>Loading...</em>
      </p>
      ) : (
        this.renderContents()
      );

    return (
      <div className="container">  
        <PageHeader />
        <NavBar onSelectNewsType={this.updateNewsItems}/>

        {contents}
      </div>
    );
  }

  renderContents = () => {
    return (
      <div>
        <div>
          <HorizontalArticleBar />
        </div>
        <div className="row">
          <div className="col-lg-7">
            {this.props.children}
          </div>
          <div className="col-lg-5">
            <VerticalNewsItemBar />
          </div>
        </div>
      </div>
    );
  }
}

const mapStateToProps = function(state) {
  return {
    newsItems: state.newsItems,
    articles: state.articles
  }
}

export default withApollo(connect(mapStateToProps)(PublicLayout));

减速器：

const initialState = {
  articles: [],
  newsItems: []
};

function RootReducer(state = initialState, action) {
  if(action.type === "REMOVE_NEWSITEMS"){
      return Object.assign({}, state, {
          newsItems: []
      });
  }
  //more here but never setting newsItems to null
};

export default RootReducer;

我发现的大多数其他类似问题是由于初始状态未初始化为非null值，但是我正在提供初始值（空数组）

编辑：我的redux商店看起来如下：

import { createStore } from "redux";
import RootReducer from "./reducers/RootReducer";

const store = createStore(RootReducer);

export default store;

Answer 1

为确保复制，我对您的代码进行了一些更改。问题出在您的减速器上。

function RootReducer(state = initialState, action) {
  if(action.type === "REMOVE_NEWSITEMS"){
      return Object.assign({}, state, {
          newsItems: []
      });
  }

  return state; // You need to return state
};

默认情况下，所有reducer都必须在任何情况下返回状态对象。即使不适合任何情况或条件。在根减速器中，您需要返回状态。

React / Redux：TypeError：无法读取未定义的属性

1 个答案: