Add config option to omit export of empty fragment subtypes

[peternedap]

Description

Adds an extra config field for omitting empty objects when using inline fragment spreads in combination with skipTypename.
This makes an upgrade from @graphql-codegen/typescript-operations version 1.18.0 to version 2 possible for people who run into problems with this. This also allows the upgrade from graphql 15 to 16, since typescript-operations 1.18 is incompatible with version 16.

Related # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)

Screenshots/Sandbox (if appropriate/relevant):

How Has This Been Tested?

Added unit tests:

• Config Should not generate empty fragments when omitEmptyFragments is set to true
• Config Should generate empty fragments when omitEmptyFragments is set to false

Test Environment:

• OS: Ubuntu
• @graphql-codegen/...: typescript-operations
• NodeJS: v17.8.0

Checklist:

• I have followed the CONTRIBUTING doc and the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Further comments

n/a

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Name	Status	Preview	Updated
graphql-code-generator	✅ Ready (Inspect)	Visit Preview	Apr 21, 2022 at 7:12AM (UTC)

[peternedap]

As a first-time contributor I'm not allowed to run the other workflows

[n1ru4l]

Could you explain why you would want to introduce a flag for generating wrong types? Omitting the types of empty fragments can cause runtime errors as the TypeScript type would no longer mirror the actual GraphQL result type.

[peternedap]

We use global unique IDs in our code base, and every queryable type implements

""" Node, to make a type queryable, it needs to implement this interface. """
interface Node {
    """ A node requires an ID. """
    id: ID!
}

Furthermore, every queryable type is queried via a spread:

""" The query root. """
type Query {
    """ Retrieves a Node through its ID. """
    node(id: ID!): Node
}

type Person implements Node {
    """ The ID of the person. """
    id: ID!

    """ Self explanatory. """
    name: String
}

queried by

export const PERSON_DATA_QUERY = gql`
    query person($id: ID!) {
        node(id: $id ) {
            ... on Person {
                id
                name
            }
        }
    }
`;

This is a flow that was well supported in old versions, but with new versions we run into the problem that the resulting generated type cannot be easily narrowed to only the person data. We'd either have to include the typename and add an extra check to every query result, or find a way to remove {} from the resulting query types.
The resulting type can indeed be mismatched runtime when a different ID than that of a person is given as a query variable, but that would also result in different errors since there'd be no Person with that ID (because of the global unique IDs).

This MR thus introduces an optional way to reintroduce support for the previously existing behaviour where empty spreads were not added to the type.

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[OscarBarrett]

Could you explain why you would want to introduce a flag for generating wrong types? Omitting the types of empty fragments can cause runtime errors as the TypeScript type would no longer mirror the actual GraphQL result type.

Please also see the example in #6873.