https://flow-cookbook.kinja.com/warning-array-access-in-flow-gives-no-warning-1839024518

arrayAccessWithFlow.js

/* @flow */
type Slide = {| id: number |}

class Test {
	slidesWithAds: Array<Slide> = [{id: 2}];
	getNonAdSlideIndex(indexWithAds: number): ?Slide {
     	return this.slidesWithAds[indexWithAds]; 
    }
}


const test = new Test();
const slide = test.getNonAdSlideIndex(0);
slide.id // < Cannot get `slide.id` because property `id` is missing in null or undefined [1].

// https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoALgTwA4FMwDKMAlgCb4C8YA3gD5hkBcYAdgK4C2ARrgE5i0AvqgDGMAIYBnSWAAquSehqoAkJJLlJAdWLoAFgEFSk5gd69xmADxEyuAHxgqAbWpMwAJkEAaGu4AsggC6ANyqAOa46ABycCxGtuQAkizkAB4AFMSpuGk6+kYmrJw8vACUzAD8ifjUqGANDSq8UWy8LGD6xJIAdOp22rqGxs7Z6fnDkqFg9Y3CwqiicYqdCkpULLhI8ooZZWEiy0r95E6rij2RMXEJGrgp6Rn++6gnuD1kQA