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Passing Asynchronously Acquired Data To Child Props

- 1 answer

I am making an app that gets an array of news items from a remote source and displays them on a page.

I have the endpoint, and can make a successful call, proven by console logs, using $.getJSON(). I placed this call into the parent component, because child components will need to use the data.

However, when I pass this data down to a child component, the console error appears:

Uncaught TypeError: Cannot read property 'headline' of undefined

This is because React is trying to render the component even before the data has been passed into it. This makes me think I should first be calling my ajax somewhere other than componentDidMount.

To get around it, I set up a method on the child component that returns the headline if the prop is present:

getHeadline: function () {
    if(this.props.newsItems){
        return this.props.newsItems.headline
    } else {
        return null
    }
},

This feels like a bit of a nasty way around it. Is there a better way, or am I missing something in my code?

var BigStory = React.createClass({

    getHeadline: function () {
        if(this.props.newsItems){
            return this.props.newsItems.headline
        } else {
            return null
        }
    },

    render: function () {
        console.log('props:', this.props);
        console.log('newsItems:', this.props.newsItems);
        return (
            <div className="big-story col-xs-12">
                <div className="col-sm-5">
                    <h1>{this.getHeadline()}</h1>
                    <p>Placeholder text here for now.</p>
                    <p>time | link</p>
                </div>
                <div className="col-sm-7">
                    <img src="http://placehold.it/320x220" alt=""/>
                </div>
            </div>
        );
    }
});

var Main = React.createClass({

    getInitialState: function () {
        return {
            newsItems: []
        }
    },

    componentDidMount: function () {
        this.getNewsItems();
    },

    getNewsItems: function () {
        $.getJSON('http://www.freecodecamp.com/news/hot', (data) => {
            console.log('data sample:', data[0]);
            this.setState({newsItems: data})
        })
    },

    render: function () {
        return (
            <div className="container">
                <div className="main-content col-sm-12">
                    <div className="left-sided-lg-top-otherwise col-lg-8 col-md-12 col-sm-12 col-xs-12">
                        <BigStory newsItems={this.state.newsItems[0]}/>
                    </div>
                </div>
            </div>
        );
    }
});
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Answer

I would suggest leaving it up to the parent to decide what to do when it is in a "loading" state and leaving BigStory as a "dumb" component that always renders the same assuming it will always receive a valid newsItem.

In this example, I show a <LoadingComponent />, but this could be whatever you need it to be. The concept is that BigStory shouldn't have to worry about edge cases with "receiving invalid data".

var Main = React.createClass({
  // ...
  render() {
    const {newsItems} = this.state;
    // You could do this, pass down `loading` explicitly, or maintain in state
    const loading = newsItems.length === 0;
    return (
      <div className="container">
          <div className="main-content col-sm-12">
              <div className="left-sided-lg-top-otherwise col-lg-8 col-md-12 col-sm-12 col-xs-12">
                  {loading 
                    ? <LoadingComponent />
                    : <BigStory newsItem={newsItems[0]} />  
                  }
              </div>
          </div>
      </div>
    );
  }
});

function BigStory(props) {
  // Render as usual. This will only be used/rendered w/ a valid
  return (
    <div className="big-story col-xs-12">
      <h1>{props.headline}</h1>
      {/* ... */}
    </div>
  )
}

An alternative solution (although I recommend an approach more like above) would be to always make use of the BigStory component in the same way, but provide it a "placeholder story" when there are no stories loaded.

const placeholderNewsItem = {
  headline: 'Loading...',
  /* ... */
};

var Main = React.createClass({
  // ...
  render() {
    const {newsItems} = this.state;
    // Conditionally pass BigStory a "placeholder" news item (i.e. with headline = 'Loading...')
    const newsItem = newsItems.length === 0
      ? placeholderNewsItem
      : newsItems[0];
    return (
      <div className="container">
          <div className="main-content col-sm-12">
              <div className="left-sided-lg-top-otherwise col-lg-8 col-md-12 col-sm-12 col-xs-12">
                  <BigStory newsItem={newsItem} />
              </div>
          </div>
      </div>
    );
  }
});
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source: stackoverflow.com
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