@graphql-tools/schema/esm/checkForResolveTypeResolver.js

import { MapperKind, mapSchema } from '@graphql-tools/utils';
// If we have any union or interface types throw if no there is no resolveType resolver
export function checkForResolveTypeResolver(schema, requireResolversForResolveType) {
    mapSchema(schema, {
        [MapperKind.ABSTRACT_TYPE]: type => {
            if (!type.resolveType) {
                const message = `Type "${type.name}" is missing a "__resolveType" resolver. Pass 'ignore' into ` +
                    '"resolverValidationOptions.requireResolversForResolveType" to disable this error.';
                if (requireResolversForResolveType === 'error') {
                    throw new Error(message);
                }
                if (requireResolversForResolveType === 'warn') {
                    console.warn(message);
                }
            }
            return undefined;
        },
    });
}