Firebase Cloud Function to get custom token login

index.js

const functions = require("firebase-functions");
const admin = require('firebase-admin');

const serviceAccount = require("./path-to-firebase-adminsdk.json");

admin.initializeApp({
  credential: admin.credential.cert(serviceAccount)
});

exports.getToken = functions.https.onCall(async (data, context) => {
  return admin
  .auth()
  .createCustomToken(data.uid)
  .then((customToken) => {
      return customToken;
    }).catch((error) => {
      return null;
    });
});