A large majority of modern web applications are built with Javascript… Both frontend and backend. You do still have a large majority of websites using plain HTML or PHP, with some features requiring JS to function (modals, realtime stats, data input, etc).

You also have alternative languages like Java or C# (and more), but also may use bits of JS on the frontend to drive functionality.

You can bet that the majority of websites you visit nowadays will use some form of JS, unless it’s a static webpage to display basic information.