Send Email Notifications to Subscribers with SendGrid and Node.js

You will need the @sendgrid/mail package which allows you to use the Twilio SendGrid API to send and receive emails. dotenv is used to access environment variables, which is where you will store your SendGrid API key needed to interact with the API. The sneaks-api package is used to track sneaker prices. The fs package will allow us to interact with our subscribers database which we will simulate using a JSON file. Lastly, the node-schedule package is used to schedule the price checker.

Once your Node.js app is initialized with the dependencies, your next step is open your preferred text editor and create three new files inside your email-notifier directory: index.js, .env, and subscribers.json.

The index.js file will continuously run and will check and notify subscribers if their sneaker has dropped below a certain threshold. The .env file will securely store all of your environmental variables. The subscribers.json will hold all of your subscribers along with their data.

Your SendGrid API key you created earlier can be securely stored in the .env file. Open up the .env file and copy and paste the following into it:

SENDGRID_API_KEY=XXXXXXX

Once copied in the .env file, replace the XXXXXXX placeholders with your actual API key.

[
 {
   "email": "you@example.com",
   "styleID": "FY2903",
   "lastResellPrice": 475
 },
 {
   "email": "you@example.com",
   "styleID": "DD0587-600",
   "lastResellPrice": 285
 }
]

The “email” key in each of the objects in the Subscribers array stores the email address of the subscriber. Make sure you enter your own email address here. The “styleID” key stores the style ID of the sneaker being tracked and the “lastResellPrice” key stores the last tracked price of the sneaker.

Your email notification system needs to send out alerts whenever the price drops for a subscriber’s tracked sneaker. The index.js file you created earlier will go through the subscribers collection and check whether the price of their tracked sneaker has dropped below a certain threshold. If the price has dropped, it will send out an email alert to the user.

At the top of the index.js file insert the following code:

require('dotenv').config()
const SneaksAPI = require('sneaks-api');
const fs = require("fs");
const sneaks = new SneaksAPI();
const schedule = require('node-schedule');
const sgMail = require('@sendgrid/mail');
sgMail.setApiKey(process.env.SENDGRID_API_KEY);
const jsonString = fs.readFileSync("./subscribers.json");
const subscribers = JSON.parse(jsonString);

This code loads the SendGrid API, and uses dotenv to load the API with your API key stored in the .env file. It also includes the sneaks-api package, the fs module and the subscribers.json file. Ensure to replace the email addresses in the subscribers file with your own email address so you’re able to test out and see price alerts.

This next section is the main function that loops through all the subscribers, grabs their current shoe price and sends out an email alert if the price has dropped; add this to the index.js file. You’ll notice references to helper functions, which you will add in the next section.

(async function main() {
   const sneakerMap = await getSneakerMap(subscribers);
   for await (const subscriber of subscribers) {
       if (sneakerMap[subscriber.styleID].price < subscriber.lastResellPrice - 10) {
           notifySubscriber(sneakerMap[subscriber.styleID], subscriber.email);
           subscriber.lastResellPrice = sneakerMap[subscriber.styleID].price;
       }
   }
   fs.writeFileSync("./subscribers.json", JSON.stringify(subscribers));
})()

The getSneakerMap() function outputs a map of all sneaker style IDs in the subscribers collection along with their current price. Once this map is generated, the code loops through all the subscribers and checks their current shoe in the map to see if their shoe price has dropped.

If it has dropped by $10, it will then run the notifySubscriber() function which sends out an alert to the user and will update the last resell price of the subscribers shoe from the sneaker map and then move on to the next subscriber. Once this loop ends, the subscribers.json file will be updated with the new prices.

async function getSneakerMap(subscribers) {
   var sneakerMap = new Object();
   for (const subscriber of subscribers) {
       if (sneakerMap[subscriber.styleID]) continue;
       const sneaker = await sneaksApiFunctionWrapper(subscriber.styleID);
       sneakerMap[subscriber.styleID] = sneaker;
   }
   return sneakerMap;
}

The notifySubscriber() function is the core of the scheduler - it will run if a subscriber needs to be notified and takes in a sneaker and an email address and sends an email alert to the address using the SendGrid API. This function uses the sgMail variable we initialized earlier with the SendGrid Node.js helper library. Instead of sending a plain text notification email, the SendGrid API provides you a way to send a stylized email with HTML. Line 6 will send a stylized HTML document to the email generated by createEmailInHTML() function.

Add notifySubscriber right after getSneakerMap in index.js.

function notifySubscriber(sneaker, email) {
   const msg = {
       to: email,
       from: SENDER_EMAIL, // Change to your verified sender
       subject: `Price Drop - ${sneaker.name}`,
       html: createEmailInHTML(sneaker),
   }
   sgMail.send(msg);
}

On line 4, replace the SENDER_EMAIL placeholder with your email.

The sneaksApiFunctionWrapper() function utilizes the sneaks-api package to format and provide the information we need. Add this function after notifySubscriber in index.js.

function sneaksApiFunctionWrapper(styleID) {
 return new Promise((resolve, reject) => {
   sneaks.getProductPrices(styleID, function (err, product) {
     const lowestResellSite = Object.keys(product.lowestResellPrice).reduce((a, b) => product.lowestResellPrice[a] > product.lowestResellPrice[b] ? a : b);
     const sneaker = {
       name: product.shoeName,
       image: product.thumbnail,
       site: lowestResellSite,
       price: product.lowestResellPrice[lowestResellSite],
       url: product.resellLinks[lowestResellSite],
       styleID: product.styleID
     };
     resolve(sneaker);
   }, (errorResponse) => {
     reject(errorResponse);
   });
 });
}

Lastly the createEmailInHTML() takes in the sneaker and generates a stylized document displaying the price drop for it as well as an image of the sneaker and a button that redirects the user to the product page.

function createEmailInHTML(sneaker) {
   return `
   <h2>
     <img style="font-size:14px;display:block;margin-left:auto;margin-right:auto;width:200px; padding-bottom:15px"
       src="https://raw.githubusercontent.com/druv5319/Sneaks-API/master/Screenshots/Sneaks_Logo.png" />
   </h2>
   <div
     style=" margin-left: auto; margin-right: auto;box-sizing:border-box;width:45%; margin-bottom:30px;background:#fdfdfd;border:1px solid #f0f0f0">
     <h2 style="text-align: center; color: black; font-family: 'avenir';">Price Drop - ${sneaker.name}</h2>
     <p>
       <img style="display: block; margin-left: auto; margin-right: auto;" src=${sneaker.image} alt="" width="auto"
         height="250" />
     </p>
     <h2 style="text-align: center; color: black; font-family: 'avenir';">$${sneaker.price}</h2>
     <p>&nbsp;</p>
     <p style="font-size: 1.5em; color: black; font-family: avenir; text-align: center;">${sneaker.name} has dropped to
       <strong>$${sneaker.price}</strong> on <strong>${sneaker.site}</strong></p>
     <p style="text-align: center;">
       <a href="${sneaker.url}"
         style="box-sizing:border-box;border-color:#348eda;font-weight:400;text-decoration:none;display:inline-block;margin:0;color:#ffffff;background-color:#348eda;border:solid 1px #348eda;border-radius:2px;font-size:14px;padding:12px 45px; font-weight: 600;">Buy
         now on ${sneaker.site}</a>
     </p>
   </div>`;
}

The highlighted lines show that the main function here is wrapped with another function called scheduleJob() which uses a cron expression as a parameter. The cron expression used is */10 * * * * and instructs the function to have it run every 10 minutes.

Save the file, run the script again with node index in the terminal and you’re done! This script will continuously run and will check if a sneaker price has dropped from the subscribers list every 10 minutes.