The Problem

When you initialize the state for your component in the constructor, be aware that the component might render using that initial state. If you don’t properly initialize the state, this can cause bugs in your code.

class Person extends Component {
    constructor(props) {
        super(props);

        this.state = {
            address: null,
        }
    }

    componentDidMount() {
        fetch("/api/v1/address").then(data => {
            this.setState({address: data})
        })
    }

    render() {
        return (
            <div>
                // This will throw an exception: "Cannot read property 'streetNumber' of null"
                <p>You live at {this.state.address.streetNumber} {this.state.address.streetName}</p>
            </div>
        )
    }
}

The Solution

This happens because a component can be rendered before componentDidMount is actually called, in which case it will be rendered using the initial value of the state. There is more than one solution to this problem. One way would be to initialize the state with dummy values:

constructor(props) {
    super(props)

    this.state = {
        address: {
            streetNumber: '?',
            streetName: '?',
        }
    }
}

The better pattern however is to have a placeholder component like a loading spinner that is rendered while waiting for the API call to return.

class Person extends Component {
    constructor(props) {
        super(props);

        this.state = {
            address: null,
        }
    }

    componentDidMount() {
        fetch("/api/v1/address").then(data => {
            this.setState({address: data})
        })
    }

    render() {
        if (this.state.address == null) {
            return (<div><p>Loading...</p></div>)
        }

        return (
            <div>
                <p>You live at {this.state.address.streetNumber} {this.state.address.streetName}</p>
            </div>
        )
    }
}

This way the user will know that the data is coming, and the component will correctly update once the data is retrieved.

The Experts

Evan Hicks

Working Code, Happy Users. Sentry Application Monitoring.

Fix Bugs Faster