Twilio function for forwarding text messages to email using mailgun.

Twilio-function-SMS-email-forward.js

let toEmail = 'TO EMAIL ADDRESS';
let fromEmail = 'FROM EMAIl ADDRESS';
let mgKey = 'INSERT MAILGUN API KEY';

exports.handler = function(context, event, callback) {
  //var mailgun = require('mailgun-js')({apiKey: context.MG_KEY, domain: context.MG_DOMAIN});
  var domain = fromEmail.split('@')[1];
  var mailgun = require('mailgun-js')({apiKey: mgKey, domain: domain});
  var number = event.From;
  
  var intCode = number.slice(0,-10);

  if(intCode) {
    intCode += ' ';
  }
  number =
    intCode +
    '(' +
    number.slice(-10,-7) +
    ') ' +
    number.slice(-7,-4) +
    '-' +
    number.slice(-4);

  var data = {
    from: event.From + '<' + fromEmail + '>',
    to: toEmail,  // This can be a list
    subject: 'New SMS from '+ number,
    text: event.Body,
    //html: '  <b>html</b> is pretty  :) \n'  + event.From + ' sent:\n' + 'event.Body' ,
  };

  mailgun.messages().send(data, function (error, body) {
    console.log(body);
  }); 
};

Thanks for the code.
I have made a couple updates:

1. Twilio has updated their requirements and a function needs to send a callback, so I am sending Mailgun's response as a callback
2. Mailgun requires an extra parameter host for their EU clients

PS: don't forget to add the NPM dependency for mailgun-js in the Configure menu in Twilio

let toEmail = 'TO EMAIL ADDRESS';
let fromEmail = 'FROM EMAIl ADDRESS';
let mgKey = 'INSERT MAILGUN API KEY';
let mgHost = 'api.mailgun.net'; // Use 'api.eu.mailgun.net' if using Mailgun EU

exports.handler = function(context, event, callback) {
  //var mailgun = require('mailgun-js')({apiKey: context.MG_KEY, domain: context.MG_DOMAIN});
  var domain = fromEmail.split('@')[1];
  var mailgun = require('mailgun-js')({apiKey: mgKey, domain: domain});
  var number = event.From;
  
  var intCode = number.slice(0,-10);

  if(intCode) {
    intCode += ' ';
  }
  number =
    intCode +
    '(' +
    number.slice(-10,-7) +
    ') ' +
    number.slice(-7,-4) +
    '-' +
    number.slice(-4);

  var data = {
    from: event.From + '<' + fromEmail + '>',
    to: toEmail,  // This can be a list
    subject: 'New SMS from '+ number,
    text: event.Body,
    //html: '  <b>html</b> is pretty  :) \n'  + event.From + ' sent:\n' + 'event.Body' ,
  };

  mailgun.messages().send(data, function (error, body) {
    console.log(body);

    let twiml = new Twilio.twiml.MessagingResponse();
    callback(error, body.message);
  }); 
};

Thank you for sharing this! Very useful!