Simple wrapper to use "googleapis" in Node

.env

HOST=mehost.com
APP_MAIL="Site Name" <memail@mehost.com>
GMAIL_APITOKEN={JSON API TOKEN}
GMAIL_APISECRET={JSON API SECRET}

gmail.js

/**
 * @licence The MIT License (MIT)
 * @copyright Copyright (C) 2017 Lauro Moraes [https://subversivo58.github.com/]
 */

const google = require('googleapis')
const googleAuth = require('google-auth-library')
const gmail = google.gmail('v1')

/**
 * Create RAW email content. Gmail API requires compatible RFC 2822 MIME and encoded as base64 strings
 * Gmail API reference @see{https://developers.google.com/gmail/api/guides/sending#creating_messages}
 * RFC 2822 @see{https://tools.ietf.org/html/rfc2822}
 * @return {string} - base64 encode email
 * @api - private
 */
const RAWMAIL = function(to, from, subject, template) {
    let content = [
        'Content-Type: text/html; charset=\'UTF-8\'\n',
        'MIME-Version: 1.0\n',
        'Content-Transfer-Encoding: 7bit\n',
        'to: ', to, '\n',
        'from: ', from, '\n',
        'subject: ', subject, '\n\n',
        template
    ].join('')

    let encodedMail = new Buffer(content).toString('base64').replace(/\+/g, '-').replace(/\//g, '_')
    return encodedMail
}

/**
 * Send
 * @return Promise
 * @api - private
 */
const sendMail = function(opts) {
    return new Promise((resolve, reject) => {
        let require = {
            auth: opts.auth,
            userId: 'me',
            resource: {
                raw: opts.raw
            }
        }
        gmail.users.messages.send(require, (err, response) => {
            if ( !!err ) {
                reject({
                    code: 1,
                    message: err
                })
            }
            resolve(response)
        })
    })
}

class Gmail {
    constructor(opts) {
        this.credentials = (!!process.env.GMAIL_APISECRET) ? JSON.parse(process.env.GMAIL_APISECRET) : false
        this.token = (!!process.env.GMAIL_APITOKEN) ? JSON.parse(process.env.GMAIL_APITOKEN) : false
        if ( !this.credentials || !this.token ) {
            this.enabled = false
        } else {
            this.enabled = true
        }
    }
    /**
     * Send function
     * @return Promise
     * @api - public
     */
    send(opts) {
        return new Promise((resolve, reject) => {
            if ( !!this.enabled ) {
                //
                if ( !opts.to || !opts.template || !opts.subject ) {
                    reject({
                        code: 0,
                        message: 'No have one or more properties "to" (email target), "template" or "subject"'
                    })
                } else {
                    // define OAuth2
                    let auth = new googleAuth()
                    let oauth2Client = new auth.OAuth2(
                        this.credentials.installed.client_id,
                        this.credentials.installed.client_secret,
                        this.credentials.installed.redirect_uris[0]
                    )
                    oauth2Client.credentials = this.token
                    // define sender (fallback to host)
                    let sender = (!!opts.from) ? opts.from : (!!process.env.APP_MAIL) ? process.env.APP_MAIL : process.env.HOST
                    // make email
                    let raw = RAWMAIL(
                        opts.to,
                        sender,
                        opts.subject,
                        opts.template // with body HTML content
                    )
                    sendMail({
                        auth: oauth2Client,
                        raw: raw
                    }).then((response) => {
                        resolve(response)
                    }).catch((e) => {
                        reject(e)
                    })
                }
            } else {
                reject({
                    code: 0,
                    message: 'No have "Gmail API Credentials"'
                })
            }
        })
    }
}

module.exports = new Gmail()

test.js

Gmail.send({
   to: 'your_email_target@domain.com',
   subject: 'Awesome subject',
   template: '<h1>Exmple of HTML</h1>'
}).then((response) => {
   //...
}).catch((e) => {
   //...
})