Narrow types for loaded normalized resources

[stephenwf]

Currently the types for ManifestNormalized and other resources do not match the IIIF specification, instead assume almost completely optional resource (for cases where a manifest, collection or annotation page has not been loaded yet). However there are times where you may know that a resource has been loaded.

Adding variations for these loaded resources: LoadedManifestNormalized with the specification constraints would allow for more accurate types. This could be a return value from vault.loadManifest() for example, and could also be used manually to narrow types.

function isLoaded(manifest: ManifestNormalized): manifest is LoadedManifestNormalized {
  return manifest.items.length > 0;
}

manifest = { ... }; // ManifestNormalized;

if (isLoaded(manifest)) {
  // manifest Narrowed to LoadedManifestNormalized.
  manifest.label; // no errors
  manifest.items[0].type; // no errors. 
}