What is the definitive way to use Gmail with OAuth and Nodemailer?

The following worked for me, there are two parts:

01) app.js

02) Google and OAuth2 setup

---

app.js

var nodemailer = require("nodemailer");

var transporter = nodemailer.createTransport({
    host: 'smtp.gmail.com',
    port: 465,
    secure: true,
    auth: {
        type: 'OAuth2',
        user: local_settings.my_gmail_username,
        clientId: local_settings.my_oauth_client_id,
        clientSecret: local_settings.my_oauth_client_secret,
        refreshToken: local_settings.my_oauth_refresh_token,
        accessToken: local_settings.my_oauth_access_token
    }
});


var mail = {
    from: "John Smith <[email protected]>",
    to: "[email protected]",
    subject: "Registration successful",
    text: "You successfully registered an account at www.mydomain.com",
    html: "<p>You successfully registered an account at www.mydomain.com</p>"
}

transporter.sendMail(mail, function(err, info) {
    if (err) {
        console.log(err);
    } else {
        // see https://nodemailer.com/usage
        console.log("info.messageId: " + info.messageId);
        console.log("info.envelope: " + info.envelope);
        console.log("info.accepted: " + info.accepted);
        console.log("info.rejected: " + info.rejected);
        console.log("info.pending: " + info.pending);
        console.log("info.response: " + info.response);
    }
    transporter.close();
});

Google and OAuth Setup

The code above requires the following setup:

01) Go to https://console.developers.google.com

02) If you don't have a project, you will be prompted to create one

03) Click on Create Project

04) Click on Create

05) Enter a Project Name and click Create

06) Select the Gmail API

07) Click on Enable

08) Click on Create Credentials

09) Enter the required settings

10) Give the OAuth client a name and ensure you add https://developers.google.com/oauthplayground as a redirect URI in order to generate the refresh and access tokens later

11) Define the consent screen settings

12) Click I'll do this later and Done

13) Click on the Edit icon, to view your Client ID and Client Secret

14) To generate access and refresh tokens, go to https://developers.google.com/oauthplayground

15) Click on the cog icon in the top right, check Use your own OAuth credentials and enter Client ID and Client Secret

16) In the left column, select Gmail API v1 and click Authorise APIs

17) If you are signed into multiple accounts, when prompted select the relevant account

18) Click Allow

19) Click Exchange authorisation code for tokens

I'm not sure why there is a count down on the access token, but hopefully the message at the bottom of the screen means that the token won't expire.

OAuth Consent Screen

You are definetely right about the gaps and outdated information, and you did a really great job on documenting the steps needed to use Gmail with OAuth and nodemailer! Nevertheless, I think it worths mentioning that in the Credentials page there is another step: the OAuth Consent Screen tab.

It contains a form like a Google Play app submission that requires validation from Google, if you choose your app to not being validated, you have a limitation of 100 calls of what they call Sensitive scopes before being asked for submission.

What about quota?

It's still not clear to me if this 100 calls quota will be consumed even if you don't select any additional permission to use sensitive scopes (the default ones are email, profile, openid). I hope not, since the OAuth Consent Screen asks for things like the Application Homepage Link and Authorised domains that is something you might not have if you are working on a backend application.

I think that this whole procedure is really slow and uselessly complex since most people do all these steps to just send an email from their app using nodemailer...