Send mail via Google Apps Gmail using service account domain wide delegation in nodejs

So I was half-step close to the solution, the problem was that while creating const jwtClient = new google.auth.JWT(googleKey.client_email, null, googleKey.private_key, ['https://www.googleapis.com/auth/gmail.send'], null); i did not mention the account to be impersonated.

The correct initialization should be: const jwtClient = new google.auth.JWT(googleKey.client_email, null, googleKey.private_key, ['https://www.googleapis.com/auth/gmail.send'], '[email protected]');

To summarize, the correct steps are:

1. Created a project in Google Cloud Platform
2. Created a service account
3. Enabled Domain Wide Delegation for the service account
4. Downloaded the key for the service account in JSON format
5. API Manager > Credentials i have created OAuth 2.0 Client ID
6. Enabled Gmail API for the project

In Google Apps Admin console:

1. In Security > Advanced Settings > Manage API client access i have added the Client ID from step 4 above
2. I have added all possible scopes for the Client ID

This is the code that sends mails:

const google = require('googleapis');
const googleKey = require('./google-services.json');
const jwtClient = new google.auth.JWT(googleKey.client_email, null, googleKey.private_key, ['https://www.googleapis.com/auth/gmail.send'], '<user to impersonate>');

jwtClient.authorize((err, tokens) => {
  if (err) {
    console.err(err);
    return;
  }
  console.log('Google auth success')
  var gmail = google.gmail({version: 'v1'})
  var raw = <build base64 string according to RFC 2822 specification>

  var sendMessage = gmail.users.messages.send({
    auth: jwtClient,
    userId: '<user to impersonate>',
    resource: {
      raw: raw
    }
  }, (err, res) => {
     if (err) {
       console.error(err);
     } else {
       console.log(res);
     }
 });

Hope that would be helpful for others