How to Receive and Respond to Incoming SMS Messages in Node.js with Express.js and Plivo

Jun 4, 2021

Sending an outbound message using the Plivo SMS platform is easy, but communication should be a two-way street. Customers should be able to text you, and you should acknowledge their messages and address their concerns. To do this, you can build an Express application to receive and respond to incoming SMS messages on a Plivo phone number. In this post, we walk you through how to implement this.

Prerequisites

Before you get started, you’ll need:

A Plivo account — sign up for one for free if you don’t have one already.
An SMS-enabled Plivo phone number as you want to receive incoming SMS messages. To search for and buy an available number, go to Phone Numbers > Buy Numbers on the Plivo console.
Express and Plivo npm packages — run npm i -S plivo express body-parser to install them.
ngrok — a utility that exposes your local development server to the internet over secure tunnels.

Create an Express application to receive SMS messages

Once you’ve installed Express and the Plivo Node.js SDK, create a simple Express application to handle incoming SMS messages on a Plivo number. Use this code:

        const express = require('express');
    const bodyParser = require('body-parser');
    const app = express();
    
    app.use(bodyParser.urlencoded({
        extended: true
    }));
    app.use(function (req, response, next) {
        response.contentType('application/xml');
        next();
    });
    app.set('port', (process.env.PORT || 3000));
    app.all('/receive_sms/', function (request, response) {
        let from_number = request.body.From || request.query.From;
        let to_number = request.body.To || request.query.To;
        let text = request.body.Text || request.query.Text;
        console.log('Message received - From: ' + from_number + ', To: ' + to_number + ', Text: ' + text);
    });
    app.listen(app.get('port'), function () {
        console.log('Node app is running on port', app.get('port'));
    });
    
    

Return a Message XML document to reply to incoming messages

To reply to an incoming SMS message, you need to return an XML document from the URL configured as the message_url in the application assigned to the Plivo number. The Node.js SDK can manage the XML document generation, and you can use the Message XML element to reply to incoming SMS messages. Use this code:

const plivo = require('plivo');
const express = require('express');
const bodyParser = require('body-parser');
const app = express();

app.use(bodyParser.urlencoded({
    extended: true
}));
app.use(function(req, response, next) {
    response.contentType('application/xml');
    next();
});
app.set('port', (process.env.PORT || 3000));
app.all('/reply_sms/', function(request, response) {
    let from_number = request.body.From || request.query.From;
    let to_number = request.body.To || request.query.To;
    let text = request.body.Text || request.query.Text;
    console.log('Message received - From: ' + from_number + ', To: ' + to_number + ', Text: ' + text);

    //send the details to generate an XML
    let r = plivo.Response();
    let params = {
        'src': to_number,
        'dst': from_number,
    };
    let message_body = "Thank you, we received your request.";
    r.addMessage(message_body, params);
    console.log(r.toXML());
    response.end(r.toXML());
});
app.listen(app.get('port'), function() {
    console.log('Node app is running on port', app.get('port'));
});

Test the code locally

Save the code in any file — we named the file reply_sms.js. To run the code on the server, go to the folder where the file resides and use the command

$ node reply_sms.js

You should see your basic server application in action on http://localhost:5000/reply_sms/.

Expose the local server to the internet using ngrok

Once you see the application working locally, the next step is to connect the application to the internet to receive and reply to messages. For that, we recommend using ngrok, which exposes local servers behind NATs and firewalls to the public internet over secure tunnels. Install it and run ngrok on the command line, specifying the port that hosts the application on which you want to receive messages (5000 in this case, as our local Expressjs application is running there):

Ngrok will display a forwarding link that you can use as a webhook to access your local server over the public network.

Test the link by opening the ngrok URL (https://31123bc8f94e.ngrok.io/reply_sms/?From=14156667777&To=14156667778) in a browser. We used HTTPie to check the XML response from the ngrok URL.

Connect the Express application to a Plivo number

The final step is to configure the app as a Plivo messaging application and assign it to a Plivo number on which you want to receive SMS messages.

Go to the Plivo console and navigate to Messaging > Applications > XML, then click on the Add New Application button in the upper right.

Provide a friendly name for the app — we used “App-Incoming-SMS” — and configure the ngrok URL https://31123bc8f94e.ngrok.io/reply_sms/ as the Message URL. Select the HTTP verb as POST, then click Create Application.

Create Plivo App to handle incoming SMS messages

Now go to Phone Numbers > Your Numbers and click on the number to which you want to assign the application. From the Plivo Application drop-down, choose the message application you just created. Finally, click Update Number.