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Getting Started With The Beacon API

Web API Series:

A tutorial of the Beacon API for web

Getting Started With The Beacon API

The Beacon API is a web API avalible for developers to implement into their application. View full list of web APIs here. The Beacon API is a Web API useful for:

  • Analytics
  • Diagnostics

Why not fetch or XMLHttpRequest?

As stated in the MDN Docs, the unfortunate truth is User Agents will typically ignore XMLHttpRequest made with an unload handler. Which is the handler we want to use before an individual leaves/navigates away from the page.


In this article we will building a very basic analytics tracking library. The objective is to:

  1. Create a backend to capture the data and serve our index.html page
  2. Track what element the end user clicked on and record it with a time stamp.
  3. Store the data that is being generated on each click to a global object to store.
  4. Before refreshing, or leaving the page send the data stored in the global object to the backend using the sendBeacon() method provided by the Beacon API.


  1. First, make sure you have Node.js installed into your system.
  2. Once Node is installed on your computer clone the project files:
git clone

or Download Repo

  1. Navigate to the start folder. WebAPIs->beaconAPI->start
  2. Open the terminal and install the dependencies npm install We've installed two dependencies. body-parser and express.
    • We will be using body-parse to parse the incoming POST request from our frontend.
    • express is the backend web framework we will be using to setup our server.

Setting Up Our Server


  • Setup a basic express server
  • Serve our index.html inside of our public/ folder to the frontend

Navigate to the server.js file in the base directory of the start/ folder.

First we want to setup our basic express server


const  express = require("express");const  app  =  express();app.listen(8081, () =>  console.log("Listening on 8081"))

In your terminal run:

node server.js

We've now successfully setup our basic express server!

When we navigate to localhost:8081 we should see the message: Cannot GET /.

What do we have to do to eliminate this error?

  1. We need to find something for our express app to do on / our base directory.
    • For example, serving our index.html to the frontend.

How do we serve our index.html from the servers side?

We are going to use express static to serve the index.html, which is a static file. Our index.html file is located in the public/ folder. When using express static we need to establish what folder we want to use to serve our static content. Since all of our static content including our index.html is inside our public folder, we are going to specify the "public" folder.


Our server.js file should look quite similar to this:

const express = require("express");const app = express();//serving the static content inside our public folderapp.use(express.static('public'))app.listen(8081, () => console.log("Listening on 8081"))

Navigate to localhost:8081/index.html in your browser.

You should now see our app.

Congratulations! You've successfully setup the server!

Setting Up Our Analytics.js File


  • Tie the analytics function on winow.onload window.unload.
  • Create an event that:
    • detects what element a user clicks on when visiting the page
    • Create a timestamp to record what time the user clicked that element
  • Store the object that holds the clicked element and the timestamp into a global object.
  • Before leaving the page send all the stored data to the backend using the Beacon API

Navigate to our analytics.js file in public/->analytics.js.

Inside our analytics.js file, the first thing we need to do is create an event handler that:

  • captures the element we clicked on inside our document
  • the time we clicked on it
  • then push it to a global array that holds the history throughout the entire end users visit to the web page.

Let's do that!

First were capturing the element we clicked on and the time we clicked on it inside our document.

document.addEventListener('click', (event) => {  {    clickedElement:,    timestamp: new Date()  };})

Next we need to create a dataHistory variable to store all data we capture from each click. Then push the captured data every time the user clicks inside the document.

let dataHistory = [];document.addEventListener('click', (event) => {  dataHistory.push({    clickedElement:,    timestamp: new Date()  });})

Now we need create our analytics function. This function will;

  • execute on window.load and window.unload
  • Check to see if our browser has the navigator.sendBeacon() method
  • If it doesn't it will simply return
  • If it does we will define the url we are sending our analytics data to the backend
  • Create a variable that will create a new Blob object that will hold our stringified JSON.
  • Send the data to the /analytics endpoint using navigator.sendBeacon()

What our analytics methods should look like:

window.onload = window.onunload = function analytics(event) {    if (!navigator.sendBeacon) return;    // Url we are sending the data to    let url = "http://localhost:8081/analytics";    //Create the data to send    const dataHistoryBlob = new Blob([JSON.stringify(dataHistory)], { type: 'application/json' });    navigator.sendBeacon(url, dataHistoryBlob);};

We've now completed our analytics.js file! This is what our file should look like!

let dataHistory = [];document.addEventListener('click', (event) => {  dataHistory.push({    clickedElement:,    timestamp: new Date()  });})window.onload = window.onunload = function analytics(event) {  if (!navigator.sendBeacon) return;  // Url we are sending the data to  let url = "http://localhost:8081/analytics";  //Create the data to send  const dataHistoryBlob = new Blob([JSON.stringify(dataHistory)], { type: 'application/json' });  navigator.sendBeacon(url, dataHistoryBlob);};

There's one last thing we haven't done. In our url variable we send the data to a backend endpoint we haven't defined yet, /analytics. Let's go to our server.js file to create that endpoint and make sure it can parse the data it's receiving.

Creating The "/analytics" Endpoint

In our server.js file let's create the /analytics endpoint.

const express = require("express");const app = express();"/analytics", (req, res) => {  console.log(req.body);  res.sendStatus(204);});app.listen(8081, () => console.log("Listening on 8081"));

If we log the data won't be able to see anything due to express. All of our data is being passed through the body of our request. Unfortunately express itself can't parse data from the body so we have to use the body-parser package from npm. Fortunately when we did an npm install setting up the project we already added the package as a dependency. Feel free to check your package.json for reference.

Now we need to add body parser to our server and use it to parse the json data being sent to the /analytics endpoint. Our file should look like this.

const express = require("express");const bodyParser = require("body-parser");const app = express();// parse application/x-www-form-urlencodedapp.use(bodyParser.urlencoded({ extended: false }));// parse application/jsonapp.use(bodyParser.json());//serving the static content inside our public folderapp.use(express.static('public'));"/analytics", (req, res) => {  console.log(req.body);  res.sendStatus(204);});app.listen(8081, () => console.log("Listening on 8081"))

If we save and restart our server we can click on several elements, refresh the page, exit the page, or navigate to a different website, we should see the data being sent and logged to our server. We have now successfully build a very simple use case for the beacon API!

Side note: To check to see what data is being sent to the backend on the browsers side. You can preserve the logs in the network tab. Picture of preserve log in the Network Tab of Chrome Developer Tools

Challenges: Expand upon the project by:

  • Separating the business logic(our click event) into our index.js file and have it import and use our analytics.js file.
  • Expand upon the current tracking capabilities by adding unique sessions.
  • Clean up the element retrieval data. We currently use .outerHTML see if there are better ways to find the elements we clicked on. :)
  • Create a pop up to give users the option to accept or decline tracking.

Happy coding!