I wrote a promise function in a firebase.js to return access key which I wanted to use everywhere after I start the server. But for some reason when I export the module and use it in app.js, I can't print the returned access_token Or a better question would be , how to use the access_token everywhere globally after the server is started.

Firebase.js is below

var {google} = require('googleapis');
var MESSAGING_SCOPE = 'https://www.googleapis.com/auth/firebase.messaging';
var SCOPES = [MESSAGING_SCOPE];
var http = require('http');

function getAccessToken() {
    return new Promise(function(resolve, reject) {
        var key = require('./serviceAccountKey.json');
        var jwtClient = new google.auth.JWT(
            key.client_email,
            null,
            key.private_key,
            SCOPES,
            null
        );
        jwtClient.authorize(function(err, tokens) {
            if (err) {
                reject(err);
                return;
            }
            resolve(tokens.access_token);
        });
   });
}

getAccessToken().then(function(access_token){

    //console.log(access_token);
    return access_token;
})

module.exports.getAccessToken = getAccessToken

Below is app.js

const express = require("express");
const ac = require('./firebase.js');
const app = express();

console.log(ac.getAccessToken);

const port = process.env.PORT || 1111 ;

app.listen(port, () => {
  //console.log(`server up and running on ${process.env.PORT}`);
  console.log(`server up and running on ` + port);
});

I start the server by nodemon index.js