ReferenceError: firebase is not defined in firebase with nodejs

I am trying to access the email and uid of current signed in user but this error is shown

const functions = require('firebase-functions');
    const admin = require('firebase-admin');
    admin.initializeApp(functions.config().firebase);
const firestore= admin.firestore();
    exports.StripeSource =functions.firestore.document('cards/{data}/tokens/{tokenid}').onCreate(async (tokenSnap,context) => {
      var user = firebase.auth().currentUser;
    var name, email, photoUrl, uid, emailVerified;
      email = user.email;
      uid = user.uid; 
    console.log('User Email is :', email);

      });

Error in fire base log

ReferenceError: firebase is not defined
    at exports.StripeSource.functions.firestore.document.onCreate (/srv/index.js:8:14)
    at cloudFunction (/srv/node_modules/firebase-functions/lib/cloud-functions.js:131:23)
    at /worker/worker.js:825:24
    at <anonymous>
    at process._tickDomainCallback (internal/process/next_tick.js:229:7)