Google OAuth using nodejs - Not able to proceed with email sending

Question

I am trying to send mail using OAuth2.0 from my gmail tp the user mail for confirming registration. The below is my code. There is no error. But no output also.

I dont have complete understanding of the code. Somehow I managed to construct them by referring so many websites.

I am stuck now. Kindly let me know the solution.

const dotenv = require("dotenv");
const nodemailer = require("nodemailer");
const oauth_link = "https://developers.google.com/oauthplayground";

const { OAuth2Client } = require("google-auth-library");
const opn = require("opn");
const destroyer = require("server-destroy");
const http = require("http");
const https = require("https");
const url = require("url");
const { google } = require("googleapis");

dotenv.config();

const { EMAIL, MAILING_ID, MAILING_REFRESH, MAILING_SECRET, MAILING_ACCESS } =
  process.env;

exports.sendVerificationEmail = async (email, name, url) => {
  console.log("inside send verification email");

  const oauth2Client = new google.auth.OAuth2(
    MAILING_ID,
    MAILING_SECRET,
    oauth_link
  );

  // Access scopes for read-only Drive activity.
  const scopes = ["https://www.googleapis.com/auth/drive.metadata.readonly"];

  // Generate a url that asks permissions for the Drive activity scope
  const authorizationUrl = oauth2Client.generateAuthUrl({
    access_type: "offline",
    scope: scopes,
    include_granted_scopes: true,
  });

  let userCredential = null;

  const server = http.createServer(async function (req, res) {
    // Example on redirecting user to Google's OAuth 2.0 server.
    if (req.url == "/") {
      res.writeHead(301, { Location: authorizationUrl });
    }
    // Receive the callback from Google's OAuth 2.0 server.
    if (req.url.startsWith("/oauth2callback")) {
      // Handle the OAuth 2.0 server response
      let q = url.parse(req.url, true).query;

      if (q.error) {
        // An error response e.g. error=access_denied
        console.log("Error:" + q.error);
      } else {
        // Get access and refresh tokens (if access_type is offline)
        let { tokens } = await oauth2Client.getToken(q.code);
        oauth2Client.setCredentials(tokens);

        /** Save credential to the global variable in case access token was refreshed.
         * ACTION ITEM: In a production app, you likely want to save the refresh token
         *              in a secure persistent database instead. */
        userCredential = tokens;

        const tokenInfo = await oAuth2Client.getTokenInfo(
          oAuth2Client.credentials.access_token
        );
        console.log("obtained tokeninfo");
        console.log(tokenInfo);

        //Sending mail
        const smtp = nodemailer.createTransport({
          service: "gmail",
          auth: {
            type: "OAuth2",
            user: EMAIL,
            clientId: MAILING_ID,
            clientSecret: MAILING_SECRET,
            refreshToken: tokenInfo.refreshToken,
            accessToken: tokenInfo.accessToken,
          },
        });

        console.log("smtp " + smtp);

        const mailOptions = {
          from: EMAIL,
          to: email,
          subject: "email verification",
          html: '<div><div style="max-width:500px;margin-bottom:1rem;align-items:center;display:flex"><img style="width:40px" src="https://seeklogo.com/images/F/facebook-icon-circle-logo-09F32F61FF-seeklogo.com.png" alt=""><span style="font-family:sans-serif;color:#3266a8;font-weight:700;padding-left:10px">Action Required : Activate your account</span></div><div style="border-top:1px solid;border-bottom:1px solid;border-color:#deb887;padding:10px;padding-bottom:20px"><div style="height:35px"><span style="font-weight:700;font-family:sans-serif">Hello ${name}</span></div><div style="height:28px;font-family:sans-serif;padding-bottom:20px"><span>You recently created a profile on our app. Please confirm your account</span></div><div style=""><a href={url} style="background-color:#3266a8;color:#fff;border-radius:10px;font-weight:700;font-family:sans-serif;text-decoration:none;font-size:15px;text-align:center;padding:9px;margin-bottom:1.5em">Confirm Your Account</a></div></div></div>',
        };
        console.log("mailOptions" + mailOptions);
        smtp.sendMail(mailOptions, (err, res) => {
          if (err) return err;
          return res;
        });
      }
    }

    // Example on revoking a token
    if (req.url == "/revoke") {
      //...
    }
    res.end();
  });
};