Recording user data for heatmap with JavaScript

The fundamental idea used by many tracking systems uses a 1x1px image which is requested with extra GET parameters. The request is added to server log file, then log files are processed to generate some statistics. So a minimalist click tracking function might look like this:

document.onclick = function(e){
  var trackImg = new Image();
  trackImg.src = 'http://tracking.server/img.gif?x='+e.clientX+'&y='+e.clientY;
}

AJAX wouldn't be useful because it is subject to same-origin policy (you won't be able to send requests to your tracking server). And you'd have to add AJAX code to your tracking script. If you want to send more data (like cursor movements) you'd store the coordinates in a variable and periodically poll for a new image with updated path in the GET parameter.

Now there are many many problems:

• cross-browser compatibility - to make the above function work in all browsers that matter at the moment you'd probably have to add 20 more lines of code
• getting useful data
  • many pages are fixed-width, centered, so raw X and Y coordinates won't let you create visual overlay of clicks n the page
  • some pages have liquid-width elements, or use a combination of min- and max-height
  • users may use different font sizes
  • dynamic elements that appear on the page in response to user's actions
• etc. etc.

When you have the tracking script worked out you only need to create a tool that takes raw server logs and turns them into shiny heatmaps :)

Heatmap analytics turns out to be WAY more complicated than just capturing the cursor coordinates. Some websites are right-aligned, some are left-aligned, some are 100%-width, some are fixed-width-"centered"... A page element can be positioned absolutely or relatively, floated etc. Oh, and there's also different screen resolutions and even multi-monitor configurations.

Here's how it works in HeatTest (I'm one of the founders, have to reveal that due to the rules):

1. JavaScript handles the onClick event: document.onclick = function(e){ } (this will not work with <a> and <input> elements, have to hack your way around)
2. Script records the XPath-address of the clicked element (since coordinates are not reliable, see above) in a form //body/div[3]/button[id=search] and the coordinates within the element.
3. Script sends a JSONP request to the server (JSONP is used because of the cross-domain limitations in browsers)
4. Server records this data into the database.

Now, the interesting part - the server.

1. To calculate the heatmap the server launches a virtual instance of a browser in-memory (we use Chromium and IE9)
2. Renders the page
3. Takes a screenshot,
4. Finds the elements' coordinates and then builds the heatmap.

It takes a lot of cpu-power and memory usage. A lot. So most of the heatmap-services including both us and CrazyEgg, have stacks of virtual machines and cloud servers for this task.