Why can a fragment tagged with `@alias` be nullable?

[waynezhang1995]

Creating issues for Relay

Questions regarding how to use Relay and/or GraphQL

We are experimenting with the new @alias directive. Given the following example:

  graphql`
          query aliasDirectiveConditionalFragmentTestQuery($skip: Boolean!) {
            viewer {
              ...aliasDirectiveTestFragment @skip(if: $skip) @alias
            }
          }
        `,

the type generated would be

 readonly viewer: {
    readonly aliasDirectiveTestFragment?: {
      readonly " $fragmentSpreads": FragmentRefs<"aliasDirectiveTestFragment">;
    } | null;
  };

while being undefined make sense but wondering why aliasDirectiveTestFragment can be null? what would be the possible scenarios where it will be null? Thank you

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[captbaritone]

I think today, in this case, I believe it will only ever return undefined. The null options is a side effect of how it's implemented. Revisiting the spec, to align with how field works, I agree that we should not return null here and update the types to specifically be just optional and not nullable.

If someone wants to explore a PR to improve this, search for FragmentAliasMetadata in https://github.com/facebook/relay/blob/71047f7398cacf9092e5f1c918635b14c7982092/compiler/crates/relay-typegen/src/visit.rs

For now I'd recommend using if(viewer.aliasDirectiveTestFragment == null) {.