title: How to get "key" prop from React element (on change)?
body: <p>I want to get option's value and key when select <code>onChange</code>.</p>
  
  <p>I know I can get option's <em>value</em> simplicity used <code>event.target.value</code> in <code>onChangeOption</code> function, but how can I get <code>key</code>?</p>
  
  <pre><code>&lt;select onChange={this.onChangeOption} value={country}&gt;
      {countryOptions.map((item, key) =&gt;
          &lt;option key={key} value={item.value}&gt;
              {item.name}
          &lt;/option&gt;
      )}
  &lt;/select&gt;
  </code></pre>