title: How to extend a parent class' instance variable in javascript
body: <p>I am trying to extend a parent class's instance variable but flow js is complaining that this is not correct. Is there something I am missing?</p>
  
  <pre><code>// BaseClass
  export type AdType = {
      dom: HTMLElement,
  };
  export default class AdsRefresh {
      ads: AdType[] = [];
  
      constructor(configs) {
          this.ads = configs;
      }
  }
  
  // ChildClass
  import type {AdType as BaseAdType, PlaceholderType} from './adsRefresh';
  
  export type AdType = {
      placeholderIndex?: number
  } &amp; BaseAdType;
  
  class AdsRefreshTiler extends AdsRefresh {
      ads: AdType[] = [];
  
      constructor(configs) {
          super(configs);
          this.ads = this.getAds(configs);
      }
  }
  
  
  </code></pre>
  
  <pre><code>Cannot extend  `AdsRefresh` [1] with `AdsRefreshTiler` because property `placeholderIndex` is missing in  `AdType` [2] but exists in  object type [3] in property `ads`.Flow(InferError)
  </code></pre>