Node script to send an email through gmail using oauth2 (requires `googleapis` and `mimetext` npm modules)

gmail-send.js

// https://console.cloud.google.com/apis/credentials/oauthclient
const CLIENT_ID = "2######################################################################m";
const CLIENT_SECRET = "G#################################K";
const REDIRECT_URL = "http://localhost:8973/google-oauth/";

const targetEmail = process.argv[2];
const subject = process.argv[3] || "This is a test from a node script";
const htmlContent = process.argv[4] || "Hello world<br/>What's <b>Up?</b>";
const senderName = "Your Name";
const senderEmail = "your-email@gmail.com";

const { google } = require("googleapis"); // auth + send
const { createMimeMessage } = require("mimetext"); // creation of email
const fs = require("fs"); // cache credentials
const url = require("url"); // interactive login
const http = require("http"); // interactive login

const SCOPES = ["https://www.googleapis.com/auth/gmail.send"];
const CREDENTIALS_CACHE_FILE = "./tokens.json";

async function loadCredentials() {
  const oauth2Client = new google.auth.OAuth2(
    CLIENT_ID,
    CLIENT_SECRET,
    REDIRECT_URL
  );

  try {
    // Try to authenticate from cache
    const tokens = JSON.parse(
      await fs.promises.readFile(CREDENTIALS_CACHE_FILE, "utf-8")
    );
    oauth2Client.setCredentials(tokens);
  } catch (err) {
    // Authenticate interactivetly
    oauth2Client.on('tokens', (tokens) => {
      fs.promises.writeFile(
        CREDENTIALS_CACHE_FILE,
        JSON.stringify(tokens, null, '  '),
        "utf-8"
      );
    });
    const urlToVisit = oauth2Client.generateAuthUrl({
      access_type: "offline",
      prompt: 'consent',
      scope: SCOPES,
    });
    console.log("Enter this url to authenticate:");
    console.log(urlToVisit);
    const oauthCode = await new Promise((resolve, reject) => {
      try {
        const server = http.createServer((req, res) => {
          if (req.url === "/favicon.ico") {
            return res.writeHead(404).end();
          }
          try {
            const searchParams = new url.URL(req.url, "http://localhost")
              .searchParams;
            const code = searchParams.get("code");
            resolve(code);
          } catch (err) {
            reject(err);
          }
          res.end("You can close this page");
          server.close();
        });
        server.on("error", reject);
        server.listen(8973);
      } catch (err) {
        reject(err);
      }
    });
    const response = await oauth2Client.getToken(oauthCode);
    const tokens = response.tokens;
    oauth2Client.setCredentials(tokens);
  }

  return oauth2Client;
}

async function init(oauth2Client) {
  const gmail = google.gmail({ version: "v1", auth: oauth2Client });

  const msg = createMimeMessage()
  msg.setSender({name: senderName, addr: senderEmail})
  msg.setRecipient(targetEmail)
  msg.setSubject(subject)
  msg.addMessage({
    contentType: 'text/html',
    data: htmlContent
  });
  const raw = msg.asRaw();
  
  const response = await gmail.users.messages.send({
    userId: "me",
    requestBody: {
      sizeEstimate: raw.length,
      raw: Buffer.from(raw).toString("base64"),
    },
  });

  if (!response || response.status !== 200 || !response.data) {
    throw new Error(`Got unexpected response: ${JSON.stringify(response)}`);
  }
  console.log(JSON.stringify(response.data));
  // {"id":"18d0ccf6xxxxxxxx","threadId":"18d0ccf6xxxxxxxx","labelIds":["UNREAD","SENT","INBOX"]}
}

loadCredentials().then(init).catch(console.error);