React Common Antipattern


Example

You should not save props into state. It is considered an anti-pattern. For example:

export default class MyComponent extends React.Component {
    constructor() {
        super();

        this.state = {
            url: ''
        }

        this.onChange = this.onChange.bind(this);
    }

    onChange(e) {
        this.setState({
            url: this.props.url + '/days=?' + e.target.value
        });
    }

    componentWillMount() {
        this.setState({url: this.props.url});
    }

    render() {
        return (
            <div>
                <input defaultValue={2} onChange={this.onChange} />

                URL: {this.state.url}
            </div>
        )
    }
}

The prop url is saved on state and then modified. Instead, choose to save the changes to a state, and then build the full path using both state and props:

export default class MyComponent extends React.Component {
    constructor() {
        super();

        this.state = {
            days: ''
        }

        this.onChange = this.onChange.bind(this);
    }

    onChange(e) {
        this.setState({
            days: e.target.value
        });
    }

    render() {
        return (
            <div>
                <input defaultValue={2} onChange={this.onChange} />

                URL: {this.props.url + '/days?=' + this.state.days}
            </div>
        )
    }
}

This is because in a React application we want to have a single source of truth - i.e. all data is the responsibility of one single component, and only one component. It is the responsibility of this component to store the data within its state, and distribute the data to other components via props.

In the first example, both the MyComponent class and its parent are maintaining 'url' within their state. If we update state.url in MyComponent, these changes are not reflected in the parent. We have lost our single source of truth, and it becomes increasingly difficult to track the flow of data through our application. Contrast this with the second example - url is only maintained in the state of the parent component, and utilised as a prop in MyComponent - we therefore maintain a single source of truth.