w3schools teaches bad practices. I'd suggest the Mozilla Developer Network instead for reference. Are we talking just vanilla JavaScript here?
If you're not already familiar with ES6, I'd recommend picking it up. Maybe do a project in React Native? :D That really teaches you some useful skills.
For C/C++, what sort of topic are you interested in? If you want to get a bit low-level in C/C++, try this:
https://scs.hosted.panopto.com/Panopto/Pages/Sessions/List.aspx#folderID=%22b96d90ae-9871-4fae-91e2-b1627b43e25e%22It's pretty challenging and definitely delves into the more complicated side of things. I'd strongly recommend getting or finding the associated CS:APP book since it contains the challenges that make the course what it is.
Can't really help with Java since I don't know of any super-challenging OOP review things out there.