feat(graphql-parse-resolve-info): add isResolveTree type guard

[Nargonath]

Fix #849

Description

This PR adds a TypeScript type guard to determine whether a value is a ResolveTree or not.

Performance impact

None (or minimal), it's just a two property read and boolean conditions.

Security impact

none

Checklist

• My code matches the project's code style and yarn lint:fix passes.
• I've added tests for the new feature, and yarn test passes.
• I have detailed the new feature in the relevant documentation.
• I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• If this is a breaking change I've explained why.

[benjie]

A safe way to know if the result is a ResolveTree or FieldsByTypeName is to know whether you called the function with keepRoot set or not. Maybe it would be better to add a function overload to parseResolveInfo, e.g.:

export function parseResolveInfo(
  resolveInfo: GraphQLResolveInfo,
  options?: Omit<ParseOptions, "keepRoot"> & { keepRoot?: false }
): ResolveTree | null | undefined;
export function parseResolveInfo(
  resolveInfo: GraphQLResolveInfo,
  options: Omit<ParseOptions, "keepRoot"> & { keepRoot: true }
): FieldsByTypeName;
export function parseResolveInfo(
  resolveInfo: GraphQLResolveInfo,
  options: ParseOptions
): ResolveTree | FieldsByTypeName | null | undefined;
export function parseResolveInfo(
  resolveInfo: GraphQLResolveInfo,
  options: ParseOptions = {}
): ResolveTree | FieldsByTypeName | null | undefined {
  // ...
}

This way you wouldn't even need the type guard, it would be typed for you automatically? If this seems like a feasible approach, please consider updating the PR to take this approach instead.

[benjie]

I think you might have mis-typed this example, we wouldn't get these results from calling graphql()?

Maybe something like this would be more helpful?

Suggested change

	const { parsedResolveInfoFragment, simplifiedFragment } = await graphql(
	Schema,
	query
	);
	const myResolver: GraphQLFieldResolver<any, any> = (source, args, context, resolveInfo) => {
	const parsedResolveInfoFragment = parseResolveInfo(resolveInfo);
	if (!isResolveTree(parsedResolveInfoFragment)) {
	throw new Error(`Expected ResolveTree`);
	}
	const simplifiedFragment = simplifyParsedResolveInfoFragmentWithType(
	parsedResolveInfoFragment,
	resolveInfo.returnType
	);
	// ...
	};

[benjie]

I'm not sure this is safe; depending on how it's used it could be tricked by a query like:

{
  field {
    name: id # `ID` type is always a string
    fieldsByTypeName: __typename
  }
}

[Nargonath]

Thanks for the suggestion @benjie. I wasn't aware of the keepRoot option. Does it already exist or is it something you suggest to create as part of this PR?

[benjie]

It already exists, IIRC it’s what controls the type that is returned. The changes I have suggested are types only.