It depends on your goal. Considering that you seem to be on a web development path, perhaps the next step could be a frameworks such as React or Angular (my preference: React).

You might also want to look into TypeScript. It should be relatively easy to learn if you already know JavaScript, considering that TypeScript is a superset of JavaScript.

Obviously you also need to at least understand CSS, but that will come with time. I would say there is no hurry.