Validate a Google client Sign in from the server side

React Client side (front):

//client functions
...
async function checkValidSignIn(decoded: any) {
try {
//your api address to validate
const endpoint = props.HOME_URL + '/api/w/after-google-signin'
console.info("endpoint: ", endpoint)
const response = await fetch(endpoint, {
method: 'POST',
headers: {
'Content-Type': 'application/json',
},
body: JSON.stringify(decoded),
})
.then(async (data) => {
// console.info('data: ', data)
const c = await data.json()
console.info('json: ', c)
if (c.success) {
alert("You are in")
} else {
alert(c.message)
googleLogout()
}
})
.catch((error) => {
const errorJson = JSON.stringify(error)
// console.info(errorJson)
alert(errorJson)
})
// setData(responseData);
} catch (error) {
console.error('Error fetching data:', error);
}
}
...

//The button
...
<GoogleLogin
onSuccess={
async function (credentialResponse: CredentialResponse): Promise<void> {
if (credentialResponse && credentialResponse.credential) {
console.log("credentialResponse: ", credentialResponse);
var decoded = jwt_decode(credentialResponse.credential);
console.log("decoded credentials: ",
JSON.stringify(decoded));
checkValidSignIn(credentialResponse);
}
}
}
onError={
function (): void {
console.log("Error happened");
return
}
}
auto_select
useOneTap
/>
...

Server side (endpoint), in this case Next-Auth

export async function processing(req: NextApiRequest,
res: NextApiResponse<Data>) {
const query = req.query
console.info("query: ", query)
console.info("body: ", req.body)
if (req.body && req.body.jti) {
} else {
res.statusMessage = "bad request"
res.status(400)
}
var token = req.body.credential
console.info("token: ", token)
const { OAuth2Client } = require('google-auth-library');
const client = new OAuth2Client(process.env.GOOGLE_CLIENT_ID);
async function verify(token: any) {
const ticket = await client.verifyIdToken({
idToken: token,
// idToken: token,
audience: process.env.GOOGLE_CLIENT_ID,  // Specify the CLIENT_ID of the app that accesses the backend
// Or, if multiple clients access the backend:
//[CLIENT_ID_1, CLIENT_ID_2, CLIENT_ID_3]
});
const payload = ticket.getPayload();
console.info("Verified!!!!!")
res.status(200).json(payload)
// const userid = payload['sub'];
}
await verify(token).catch((error) => {
console.log("*****error adminInitiateAuth: ", error)
console.log(JSON.stringify(error))
res.status(500).json({ msg: "no" })
})