Single function Twilio Fax handling

fax-to-email.js

/**
 * !! Requires a SendGrid account & API Key.
 * Sign up for the FREE account and create an API key.
 * If you're receiving more than 100 faxes a day you
 * shouldn't be entertaining the idea of a script like this...
 *
 * In the Function Instance view:
 * - Paste this entire script
 * - Set the url path for your Function to: /fax-to-email
 * 
 * In the global functions Configure screen:
 * - Ensure that "Enable ACCOUNT_SID and AUTH_TOKEN" is selected
 * - Include `got` version `8.3.1` in Dependencies
 * - Add environment variables and values for:
 *   - FAX_EMAIL_FROM
 *   - FAX_EMAIL_FROM_NAME
 *   - FAX_EMAIL_TO
 *   - SENDGRID_API_SEND_ONLY
 */

const got = require('got');

exports.handler = function(context, event, callback) {
    const faxBaseUrl = `https://${context.DOMAIN_NAME}/fax-to-email`;
    
    function redirectToMedia() {
        let res = new Twilio.Response();
	
    	context.getTwilioClient().fax.v1.faxes(event.f).fetch().then((data) => {
    	   res.setStatusCode(302);
    	   res.appendHeader('Location', data.mediaUrl);
    	   callback(null, res);
    	}).catch((err) => {
    	    console.log(err);
    	    res.setStatusCode(500);
    	    res.setBody(err);
    	    callback(null, res);
    	});
    }
    
    function receiveRedirect() {
        const redirect = `
            <Response>
                <Receive action="${faxBaseUrl}"/>
            </Response>
        `.trim();
        callback(null, redirect);
    }
    
    function receivedSendEmail() {
    	const emailBody = `You've got a new fax from "${event.From}". Link: ` +
    	    `${faxBaseUrl}?f=${event.FaxSid}`;
    
        const requestBody = {
            personalizations: [{
                to: [{
                    email: context.FAX_EMAIL_FROM,
                    name: context.FAX_EMAIL_FROM_NAME
                }],
                subject: `New fax from ${event.From}`
            }],
            from: {
                email: context.FAX_EMAIL_TO,
                name: 'Magic Fax Receiver'
            },
            content: [{
                type: 'text/plain',
                value: emailBody
            }]
        };
        
        const request = {
           headers: {
               Authorization: `Bearer ${context.SENDGRID_API_SEND_ONLY}`,
               'Content-Type': 'application/json'
           },
           body: JSON.stringify(requestBody)
        };
        
        got.post('https://api.sendgrid.com/v3/mail/send', request).then((response) => {
            callback(null, new Twilio.twiml.MessagingResponse());
        }).catch((err) => {
            console.log(err);
            callback(err);
        });        
    }

    switch (true) {
	// not terribly scientific, but its good enough
        case (event.f && event.f.length === 34):
            redirectToMedia();
            break;
        case (event.Status && event.Status === 'received'):
            receivedSendEmail();    
            break;
        default:
            receiveRedirect();
    }    
};