How does a web page determine if/when something is viewed?

Question

I'm seeking to understand the code and function(s) that determines when a particular part of a webpage has been viewed (for mere understanding).

For example, on the Stack Exchange web sites, it determines when a person has fully read the "about" page (and awards a badge for such). What function/code/language is watching for this?

Or at least, how could I trace and/or determine where this function is in a page's source?

Answer 1

You can use JavaScript to detect how far a user has scrolled on the page, and if they've scrolled past a certain point (say 80% of the article) you can fire off an AJAX request which logs that the user has scrolled past that point.

Quick example in jQuery:

$('body').scroll(function () {
    if($(this).scrollTop() > 1000) {
        // Fire off AJAX request
    }
});

Didn't test that, and you'll want to change 1000 to be the amount of pixels until you get to an element in the article so it works for different article lengths.