firebase cloud functions failed to access google sheet api - firebase

Question

I want to build an api service that use google sheet api to create, read and write the spreadsheets. The api service is deployed on firebase cloud functions with express.js, I have created the service account, and use JWT for authentication. The api works (i.e. can create, read and write spreadsheets) when test locally, but fails when I deployed to firebase cloud functions.
const functions = require('firebase-functions');
// firebase admin
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);
// google sheet api
const google = require('googleapis');
const key = require('./key/serviceKey.json');
const scopes = [
'https://www.googleapis.com/auth/spreadsheets',
'https://www.googleapis.com/auth/drive'
];
const authClient = new google.auth.JWT(
key.client_email, null, key.private_key, scopes, null
);
authClient.authorize((err, tokens) => {
if (err) {
console.log('error occurred when authorising with JWT');
} else {
console.log('auth success, tokens', tokens);
google.options({ auth: authClient });
}
});
const drive = google.drive('v2');
const sheets = google.sheets('v4');
The error message I get when I test it on firebase cloud functions:
Request is missing required authentication credential. Expected OAuth
2 access token, login cookie or other valid authentication credential.
Ultimately, I will have an ionic app that calls the api to perform certain actions (e.g. create and share spreadsheets with other user). So am I doing the right approach or I should use other types of authentication?
My function definitions:
const functions = require('firebase-functions');
const express = require('express');
const cors = require('cors')({ origin: true });
const bodyParser = require('body-parser');
const app = express();
app.use(cors);
app.use(bodyParser.json({ type: 'application/json' }));
// some routing setup
exports.api = functions.https.onRequest(app);
Router setup:
const express = require('express');
const router = express.Router();
router.post('/createSheets', (req, res) => {
const request = { // }
sheets.spreadsheets.create(request, (err, sheetsResponse) => {
// some code
}
});