When bind is not set, why `this` is not referring to the window object?
<p>I am wondering why if I modify this line <code>{this.increaseQty.bind(this)}</code> to <code>{this.increaseQty}</code>, the console prompts <strong>Uncaught TypeError: Cannot read property 'setState' of undefined</strong> instead of <strong>Uncaught TypeError: this.setState is not a function(…)</strong> ?? If the bind is not set, shouldn't <code>this</code> be the window object ?</p>

<pre><code>export default class CartItem extends React.Component {
    constructor(props) {
        super(props);
        this.state = {
            qty: props.initialQty,
            total: 0
        };
    }
    componentWillMount() {
        this.recalculateTotal();
    }
    increaseQty() {
        this.setState({qty: this.state.qty + 1}, this.recalculateTotal);
    }
    decreaseQty() {
        let newQty = this.state.qty &gt; 0 ? this.state.qty - 1 : 0;
        this.setState({qty: newQty}, this.recalculateTotal);
    }
    recalculateTotal() {
        this.setState({total: this.state.qty * this.props.price});
    }
    render() {
        return &lt;article className="row large-4"&gt;
            &lt;figure className="text-center"&gt;
                &lt;p&gt;
                    &lt;img src={this.props.image}/&gt;
                &lt;/p&gt;
                &lt;figcaption&gt;
                    &lt;h2&gt;{this.props.title}&lt;/h2&gt;
                &lt;/figcaption&gt;
            &lt;/figure&gt;
            &lt;p className="large-4 column"&gt;&lt;strong&gt;Quantity: {this.state.qty}&lt;/strong&gt;&lt;/p&gt;

            &lt;p className="large-4 column"&gt;
                &lt;button onClick={this.increaseQty.bind(this)} className="button success"&gt;+&lt;/button&gt;
                &lt;button onClick={this.decreaseQty.bind(this)} className="button alert"&gt;-&lt;/button&gt;
            &lt;/p&gt;

            &lt;p className="large-4 column"&gt;&lt;strong&gt;Price per item:&lt;/strong&gt; ${this.props.price}&lt;/p&gt;

            &lt;h3 className="large-12 column text-center"&gt;
                Total: ${this.state.total}
            &lt;/h3&gt;

        &lt;/article&gt;;
    }
}
</code></pre>