Do you need Java Script still or can Flask take its place?

The short answer-- you can definitely make good serviceable websites without JavaScript using Flask. Especially if you have some mastery of CSS and HTML.

Long answer-- If you don't include JavaScript, you end up having to perform page-reloads when there is a state-change. That may be fine for your purposes. But when you want pages to appear "live" or interact with more complex user mouse clicks and key-presses, or where you need partial rendering in-place without having to reload a page-- that is JavaScript territory. For example, suppose you want to show a list of stocks in a portfolio (with Flask) you can only grab the prices of the stocks just before you call flask.render_template() -- this works. But... if we add some clever JavaScript in an event-loop, the page could update prices every 2 seconds and make it look like a live-feed to the user.

In the real world, you're going to find JavaScript necessary for some client-side interactivity and validation. Things like drag-and-drop uploads, rich-text editors, live dash-boards, interactive graphs are client-side JavaScript. But like most modern programming, you can get this functionality from third-party libraries and not have to "re-invent the wheel."

There are great resources on Treehouse for learning a whole track of JavaScript. As a Python web programmer I would recommend watching these and maybe even do a bit of JavaScript server-side programming in Express/Node.js. It will make you a better web programmer and open you up to new worlds, and possibilities in web design.

I still use javascript for certain things since it is client side. I use jquery to make API calls to my flask server, which will then return content. I also use HTML and CSS quite a bit, flask is not a replacement for these things.