The Problem

Class functions are not bound to the class instance automatically, so if you attempt to use a class function that references this as a prop in another component, you will get an error.

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

        this.state = {
            name: "",
        }
    }

    handleNameChange(evt) {
        this.setState({name: evt.target.value})
    }

    render() {
        return (
            <div>
                <input onChange={this.handleNameChange} value={this.state.name} />    
            </div>
        )
    }
}

When the input is changed and handleNameChange is called, it will throw an exception because it doesn’t have the correct reference to this.

The Solution

There are a lot of ways to fix this. It is even possible to install plugins/helpers that will fix this problem for you. An example of a Babel plugin that will fix this is here: https://babeljs.io/docs/en/babel-plugin-proposal-class-properties.

One way of fixing this problem is to use a wrapper arrow function around the class method, so that the correct binding is maintained.

render() {
    return (
        <div>
            <input onChange={evt => this.handleNameChange(evt)} value={this.state.name} />    
        </div>
    )
}

Another way to address the issue is to bind the functions to the correct class instance when the class is first initialized.

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

        this.state = {
            name: "",
        }

        this.handleNameChange.bind(this)
    }

    handleNameChange(evt) {
        this.setState({name: evt.target.value})
    }

    render() {
        return (
            <div>
                <input onChange={this.handleNameChange} value={this.state.name} />    
            </div>
        )
    }
}

Now the class function will be correctly bound to the class instance when it’s constructed, and there will be no issues with referencing this anywhere in the class function.

The Experts

Evan Hicks

Working Code, Happy Users. Sentry Application Monitoring.

Fix Bugs Faster