JavaScript makes the code go 'round
JavaScript is a complex, nested language that runs via an interpreter once only found in the browser, but has been ported outside the browser by Google's V8 engine in the form of NodeJS. This allows JS to be used on the server backend processes as well as the client-side frondend. This solidifies the use of JSON as the data media type of choice to move data back and forth.
The orginal intent was to allow data switch-out without having to reload the page. New information could be swapped out on the screen from preloaded files being brought from the server when the page was loaded. Since then, animation and subtle manipulation of the DOM increases the user experience a great deal.
JS has had a long series of extensive updates, which causes a lot of confusion about how to use the language. The more recent improvement are very important and will totally subplant older technique so caution should be taken when first learning the language. Start with the latest and wink and nod at the past.