flow-verifier.js

// from this

type SavedPlaylistItem = {
  id: string,
  checksum: number,
};

// generate this

function getSavedPlaylistItem(payload: Object): ?SavedPlaylistItem {
  if (
    typeof payload.id === 'string' &&
    typeof payload.checksum === 'number'
  ) {
    return {
      id: payload.id,
      checksum: payload.checksum,
    };
  }
  return null;
}