React – uncaught TypeError: Cannot read property ‘setState’ of undefined

React – uncaught TypeError: Cannot read property ‘setState’ of undefined

I am getting the following error

Uncaught TypeError: Cannot read property ‘setState’ of undefined

even after binding delta in the constructor.
class Counter extends React.Component {
constructor(props) {
super(props);

this.state = {
count : 1
};

this.delta.bind(this);
}

delta() {
this.setState({
count : this.state.count++
});
}

render() {
return (

{this.state.count}

);
}
}

Solutions/Answers:

Solution 1:

This is due to this.delta not being bound to this.

In order to bind set this.delta = this.delta.bind(this) in the constructor:

constructor(props) {
    super(props);

    this.state = {
        count : 1
    };

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

Currently, you are calling bind. But bind returns a bound function. You need to set the function to its bound value.

Solution 2:

In ES7+ (ES2016) you can use the experimental function bind syntax operator :: to bind. It is a syntactic sugar and will do the same as Davin Tryon’s answer.

You can then rewrite this.delta = this.delta.bind(this); to this.delta = ::this.delta;


For ES6+ (ES2015) you can also use the ES6+ arrow function (=>) to be able to use this.

delta = () => {
    this.setState({
        count : this.state.count + 1
    });
}

Why ? From the Mozilla doc :

Until arrow functions, every new function defined its own this value […]. This proved to be annoying with an object-oriented style of programming.

Arrow functions capture the this value of the enclosing context […]

Solution 3:

There is a difference of context between ES5 and ES6 class. So, there will be a little difference between the implementations as well.

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Here is the ES5 version:

var Counter = React.createClass({
    getInitialState: function() { return { count : 1 }; },
    delta: function() {
        this.setState({
            count : this.state.count++
        });
    },
    render: function() {
        return (
            <div>
              <h1>{this.state.count}</h1>
              <button onClick={this.delta}>+</button>
            </div>
            );
    }
});

and here is the ES6 version:

class Counter extends React.Component {
    constructor(props) {
        super(props);
        this.state = { count : 1 };
    }

    delta() {
        this.setState({
            count : this.state.count++
        });
    }

    render() {
        return (
            <div>
              <h1>{this.state.count}</h1>
              <button onClick={this.delta.bind(this)}>+</button>
            </div>
            );
    }
}

Just be careful, beside the syntax difference in the class implementation, there is a difference in the event handler binding.

In the ES5 version, it’s

              <button onClick={this.delta}>+</button>

In the ES6 version, it’s:

              <button onClick={this.delta.bind(this)}>+</button>

Solution 4:

When using ES6 code in React always use arrow functions, because the this context is automatically binded with it

Use this:

(videos) => {
    this.setState({ videos: videos });
    console.log(this.state.videos);
};

instead of:

function(videos) {
    this.setState({ videos: videos });
    console.log(this.state.videos);
};

Solution 5:

You dont have to bind anything, Just use Arrow functions like this:

class Counter extends React.Component {
    constructor(props) {
        super(props);

        this.state = {
            count: 1
        };

    }
    //ARROW FUNCTION
    delta = () => {
        this.setState({
            count: this.state.count++
        });
    }

    render() {
        return (
            <div>
                <h1>{this.state.count}</h1>
                <button onClick={this.delta}>+</button>
            </div>
        );
    }
}

Solution 6:

You can also use:

<button onClick={()=>this.delta()}>+</button>

Or:

<button onClick={event=>this.delta(event)}>+</button>

If you are passing some params..

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