Client-side tracking

Definition

Client-side tracking is the traditional method of web data collection. JavaScript scripts run in the user’s browser and send data directly to the servers of analytics and advertising tools. It is the default mode for GA4, GTM and most marketing pixels.

How it works

When a visitor loads a page, the browser executes the JavaScript tags present on it. Each tag collects information (URL, page title, event parameters) and sends HTTP requests to collection endpoints (google-analytics.com, facebook.com, etc.). Cookies store identifiers that allow the user to be recognized across visits.

Limitations

Client-side tracking faces growing obstacles. Ad blockers (uBlock, Brave) block requests to known tracking domains. Browsers (Safari ITP, Firefox ETP) limit cookie lifespan and block third-party cookies. The number of scripts degrades page performance. And each tag adds a potential attack vector for security.

Client-side vs server-side

Server-side tracking addresses most of these limitations. However, client-side remains necessary for certain data collection tasks (JavaScript interactions, DOM event detection). The best approach combines both: a lightweight client that sends data to your server, and the server redistributes to third-party tools.