I would choose any known CMS and write plugins for it. Just look what others are doing and build it. You can make it also only for yourself.

Important is that you simply start and try out. If you have questions, you can fill Google with it. Stackoverflow is filled with solutions.

For example, you could write a review plugin for WordPress and rate websites on your blog. Or something similar. There are enough things you can do.

In any case: Just start!

PS: You can also simply help others and expand your skills.

If you are just starting JavaScript then projects are not recommended. I recommend participating in coding challenges like in hacker rank etc.

If you are pretty good with the basics of JavaScript. Then try mocking websites you visit daily. Like Netflix or YouTube or any blog website.

If you using frameworks then try adding new features that would look good in your favorite websites. Like mock the website then add extra features that have some dynamically changing data from the backend.

I hope this answers your question.