title: Add types for listener variables react-native typescript
body: <p>I have a stateful class and I need to add a listener (specifically for the keyboard). In its current form, I get an error: </p>
  
  <blockquote>
    <p>TS2339 property keyboardDidShowListener doesn't exist on type 'Auth'</p>
  </blockquote>
  
  <p>I would like to know where and what type of declaration I need to put in to stop the error.</p>
  
  <p>Here's part of the code, let me know if you need more.</p>
  
  <pre><code>class Auth extends Component&lt;Props, any&gt; {
      componentDidMount() {
          this.keyboardDidShowListener = Keyboard.addListener('keyboardDidShow', this.keyboardDidShow)
      }
  
     componentWillUnmount () {
         this.keyboardDidShowListener.remove()
     }
  .....rest of class
  </code></pre>