getUserByInitialEmail.js

const functions = require('firebase-functions');
const admin = require('firebase-admin');
const axios = require('axios');
admin.initializeApp();

async function adminAuthEmailRequest(request) {
  var token = await admin.apps[0].INTERNAL.getToken();
  var requestCopy = JSON.parse(JSON.stringify(request));
  requestCopy.headers = requestCopy.headers || {};
  var authHeader = 'Authorization';
  requestCopy.headers[authHeader] = 'Bearer ' + token.accessToken;
  console.log(JSON.stringify(request));
  return axios(requestCopy);
}

async function initialEmailLookup(email) {
  var token = await admin.apps[0].INTERNAL.idToken;
  const request = {
    url: 'https://www.googleapis.com/identitytoolkit/v3/relyingparty/getAccountInfo',
    data: {
      idToken: token,
      initial_email: email,
    },
    headers: {
      'Content-Type': 'application/json',
    },
    method: 'POST',
  };

  const response = await adminAuthEmailRequest(request);
  return response;
}

exports.checkInitialEmail = functions.https.onRequest((req,res) => {
  initialEmailLookup(req.body.email)
    .then(data => {
      console.log(data.data);
      res.status(200).send(data.data);
    })
    .catch(err => {
      console.log('ERROR:::::', err);
      res.status(400).send(err);
    });
});