JavaScript is what makes the browser interactive - it allows manipulation of every HTML element and every CSS attribute. These days, though, more and more things like animations are also possible with CSS, so JavaScript is no longer needed for that. However, aspects that require more complex logic still rely on JavaScript.
Now, there is a variety of frameworks for JavaScript, with the purpose of requiring less code to accomplish more - Angular, React, Vue, jQuery and many more. Each one has its justification for existence, they are all good and overall useful, but I personally use and prefer jQuery. Mainly for the reason that it is rather low-level in direct comparison and it also provides cross-browser compatibility - the latter is something many developers are not aware of at all.
Leaving regular websites aside completely, however, there is something else possible with JavaScript: you can use HTML, CSS and JavaScript to dynamically generate design content - for example, name tags for company employees, which you then print out and cut to size. Or even for business cards. This way, customers don't have to rely on a designer for every little change. And these kinds of solutions also work without Internet access: by packaging everything in an ".html" file that is made available to the customer so that he/she can simply drag and drop it into the browser.
Now, there is a variety of frameworks for JavaScript, with the purpose of requiring less code to accomplish more - Angular, React, Vue, jQuery and many more. Each one has its justification for existence, they are all good and overall useful, but I personally use and prefer jQuery. Mainly for the reason that it is rather low-level in direct comparison and it also provides cross-browser compatibility - the latter is something many developers are not aware of at all.
Leaving regular websites aside completely, however, there is something else possible with JavaScript: you can use HTML, CSS and JavaScript to dynamically generate design content - for example, name tags for company employees, which you then print out and cut to size. Or even for business cards. This way, customers don't have to rely on a designer for every little change. And these kinds of solutions also work without Internet access: by packaging everything in an ".html" file that is made available to the customer so that he/she can simply drag and drop it into the browser.