https://github.com/googleapis/nodejs-pubsub/blob/db1f69c4aac5b3abfea12f70d45ceb8b7c957388/src/v1/subscriber_client.ts#L2186

It seems that there is an error in SubsriberClient.testIamPermissions method return type. It says that the return type is IamProtos.google.iam.v1.TestIamPermissionsResponse but the method returns an array where the first element is IamProtos.google.iam.v1.TestIamPermissionsResponse.