This is an excellent (and free) online tutor for just about anything, provided by Contact North, an online learning organization based in Ontario. I don't recommend using it if you know absolutely nothing about a topic but, as long as you have even a very loose grasp of of the basics, it will help you to confirm and build confidence in what you know and it will help you to learn more.

Try "JavaScript programming" in the search field and it will launch a series of questions, usually fairly easy at first but introducing more complex topics as you answer them. If you get the answer wrong then it tells you why, and it will provide an answer if you type "I don't know". It is smart enough to understand almost-right and incomplete answers, and it seems pretty good at interpreting variations on the expected results so, for instance, you can use whatever variable names you like and it will know what you mean.

This will work for just about any of the skills required for this course, so do try different search terms if you want to enhance your skills for any element.

Well worth spending some time on.