React - uncaught TypeError: Cannot read property 'setState' of undefined

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 [...]

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.

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

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>