Cloud Functions + TypeScriptでserviceAccountの設定で詰まった時のとてもシンプルな対処法

結論：　ServiceAccountの型を付与する

firebase.ts

import * as admin from 'firebase-admin';
import { ServiceAccount } from 'firebase-admin';
import serviceAccount from '../serviceAccountKey.json';

admin.initializeApp({
  credential: admin.credential.cert(serviceAccount as ServiceAccount),
 // 上に記したように `ServiceAccount`の型を与える
});

export const firestore = admin.firestore();

---

型を与えないと下記のようなエラーが表示されます

Argument of type '{ type: string; project_id: string; private_key_id: string; private_key: string; client_email: string; client_id: string; auth_uri: string; token_uri: string; auth_provider_x509_cert_url: string; client_x509_cert_url: string; }' is not assignable to parameter of type 'string | ServiceAccount'.
  Type '{ type: string; project_id: string; private_key_id: string; private_key: string; client_email: string; client_id: string; auth_uri: string; token_uri: string; auth_provider_x509_cert_url: string; client_x509_cert_url: string; }' has no properties in common with type 'ServiceAccount'.

ソース：
https://github.com/firebase/firebase-admin-node/issues/522
https://firebase.google.com/docs/reference/admin/node/admin.ServiceAccount