Top 10 JavaScript Errors From 1000+ Projects (and How to Avoid Them)

class Quiz extends Component {
  // Added this:
  constructor(props) {
    super(props);

    // Assign state itself, and a default value for items
    this.state = {
      items: []
    };
  }

  componentWillMount() {
    axios.get('/thedata').then(res => {
      this.setState({items: res.data});
    });
  }

  render() {
    return (
      <ul>
        {this.state.items.map(item =>
          <li key={item.id}>{item.name}</li>
        )}
      </ul>
    );
  }
}

The exact code in your app might be different, but we hope we’ve given you enough of a clue to either fix or avoid this problem in your app. If not, keep reading because we’ll cover more examples of related errors below.

2. TypeError: ‘undefined’ Is Not an Object (evaluating…)

This is an error that occurs in Safari when you read a property or call a method on an undefined object. You can test this very easily in the Safari Developer Console. This is essentially the same as the above error for Chrome, but Safari uses a different error message.

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