This seems very similar to codingbat.com. My CS teacher in high school made me do codingbat and it was very helpful; I could see this site doing the same for JS learners. Codingbat is not as mobiles optimized but is still usable on my phone. As another commenter mentioned, some important differences are:

Number of problems, sorted into levels of difficulty / focus areas (this has only one problem that I would consider 'hard')

Help (not just solution) is available to those who want it

Helpful error messages

Syntax highlighting

That said, I really like the interface here. It works well on my phone and looks nice on a desktop, and is straightforward and simple. The only issue is that keyboard auto capitalization/autocorrect gets annoying to deal with on a phone. Hacker's Keyboard (free software Android keyboard) could help with that; it would be nice to have a link to something like that for Android+iOS on the website.