This is going to take O(N²): for each field to check, includes() will iterate the whole fieldsToChangeWithFunc. If you change fieldsToChangeWithFunc to a Set<string>, the .has() check will be faster.

cool!

Let's make requestFields required. I think transcode() is only called from here, and it's completely reasonable to assume this parameter is always required.

transcode is not exported publicly so it's an internal change that won't be breaking.

This would break a lot of tests, so it implies a different expected behavior. Is there a specific reason you want to make it required?

Nope, just to save on some if statements. It's ok to leave it as is.

This all can be shortened to (fields?.[field]?.resolvedType as Type)?.fields

actually, something like

const fields = (fields?.[field]?.resolvedType as Type)?.fields;
if (fields) {
  getAllFieldNames(fields, fieldNames);
}

The set should be built here, otherwise, you keep making the set from the list on each invocation of the recursive function. You can just do it once here and pass it to the requestChangeCaseAndCleanup.

@alexander-fenster, Set does not have the map function we use in line 246. So our options are to either take a Set as a parameter and iterate through the elements with a loop and reassign, or else what we have here - what would you prefer?

The size check is technically not needed, .has() is a no-op for an empty set.

This was breaking tests, because it would enter this comparison if there was an empty set, and, since it would find no match, this would evaluate to true, instead of false: !fieldsToChange?.has(convertedField)

thus reverting convertedField back to field.

Are we actually using this function anywhere else? If not, let's not export it.