title: Reactjs state while value setted throw undefined error
body: <p>in <strong>handleSelectChange</strong> the <strong><em>console.log(this.state.selectValue);</em></strong> print's the value but when <strong>addProduct</strong> executed throw error that <strong><em>this.state.selectValue</em></strong> is undefined</p>
  
  <blockquote>
    <p>Cannot read property 'selectValue' of undefined</p>
  </blockquote>
  
  <pre><code>  handleSelectChange(e){
      console.log(e.target.value);
      this.setState({selectValue: e.target.value});
      console.log(this.state.selectValue); //here ok
    }
  
    addProduct(e){
      console.log(this.state.selectValue); //here error
      this.setState({products: [ ...this.state.products, this.state.selectValue]});
  
    }
  
  &lt;FormGroup&gt;
       &lt;Input type="select" value={this.state.selectValue} onChange={this.handleSelectChange.bind(this)} name="select" id="exampleSelect"&gt;
          &lt;option value="Product 1"&gt;Product 1&lt;/option&gt;
          &lt;option value="Product 2"&gt;Product 2&lt;/option&gt;
          &lt;option value="Product 3"&gt;Product 3&lt;/option&gt;
          &lt;option value="Product 4"&gt;Product 4&lt;/option&gt;
          &lt;option value="Product 5"&gt;Product 5&lt;/option&gt;
        &lt;/Input&gt;
        &lt;Input placeholder="Quantity" type="number" name="posotita" id="posotita" /&gt;
        &lt;Button color="primary" onClick={this.addProduct}&gt;Add&lt;/Button&gt;
  &lt;/FormGroup&gt;
  </code></pre>