This is a specific document use of reactive programming, a paradigm where values are automatically updated when their dependencies change. Flapjax has offered reactive javascript for a long time now: http://www.flapjax-lang.org/

I think Flapjax didn't gain much traction largely because the website is ugly. Of course the the other reason is that regular event binding tends to be a good enough solution.

Some of the new client-side javascript solutions like knockout.js also offer reactive capabilities.