Flow error when using react-apollo Query component render prop - flowtype

Question

I have the following code:
import { Query } from 'react-apollo';
type Post = {
id: string
};
interface Data {
posts: Array<Post>;
}
class PostsQuery extends Query<Data> {}
When using the above as follows:
<PostsQuery query={POSTS_QUERY}>
{({ loading, data }) => {
...
{data.posts.map(...)}
...
}
</PostsQuery>
I get the following error from flow:
Error:(151, 27) Cannot get 'data.posts' because property 'posts' is missing in object type [1].
Any idea why?
I did use flow-typed to add apollo-client_v2.x.x.js to my project by the way

Answer 1

Solution to the problem
Continued from the answer explaining how to make a verifiable example and research the problem.
So it looks like this part of react-apollo isn't typed in such a way to make accessing the data contents straightforward. Okay, that's fine, we can take their recommendation on destructuring and check for empty data. At the same time, we can also add an id property to the Post type so flow stops complaining about that:
(Try - Scroll to bottom for relevant code)
type Post = {
id: string,
title: string;
};
...snip...
// Look ma, no errors
class Search extends React.Component<{}> {
render = () => (
<PostsQuery query={QUERY}>
{({ loading, error, data }) => {
if (error) {
return <p>Error</p>
}
if (loading) return <p>Loading</p>
const nonNullData = (data || {})
const dataWithAllPosts = {allPosts: [], ...nonNullData}
const {allPosts} = dataWithAllPosts
if (allPosts.length == 0) {
return <p>Empty response or something</p>
}
return (
<div>
{allPosts.map(post => {
return <div key={post.id}>{post.title}</div>;
})}
</div>
);
}}
</PostsQuery>
);
}
I'm not familiar with the react-apollo library, so I'm not sure how you want to handle that case where there are no posts. I just added a message as seen above. It's entirely possible that the case never occurs (again, you would know better than I do). If that's the case, you might want to skip some of the above steps and just assert the desired type with a cast through any.

Answer 2

How to make a reproducible example and research the problem
So the first thing we need to do while analyzing these types is to go lookup the typedefs in the flow-typed repo. I went ahead a copy-pasted the react-apollo typedefs into flow.org/try, modified them slightly (added an any somewhere, set gql to any), and was able to replicate your errors:
(Try - Scroll to the bottom for your code)
Referencing the relevant lines of the QueryRenderProps type, we can see why flow is throwing the error:
{
data: TData | {||} | void,
...
}
It looks like data can either be TData (probably what you want), an empty object, or undefined. Cross checking this with the typescript typings for react-apollo, we can see why the type is the way it is:
{
...
// we create an empty object to make checking for data
// easier for consumers (i.e. instead of data && data.user
// you can just check data.user) this also makes destructring
// easier (i.e. { data: { user } })
// however, this isn't realy possible with TypeScript that
// I'm aware of. So intead we enforce checking for data
// like so result.data!.user. This tells TS to use TData
// XXX is there a better way to do this?
data: TData | undefined;
...
}
Unfortunately, due to the extreme length of these links and stackoverflow's limit on answer lengths, I have to continue my answer in another answer. I guess this answer can serve as an explanation of how to start debugging the problem.