A few years ago I released the js-assessment project, aimed at helping developers get a sense of their knowledge of JavaScript fundamentals. There are a couple of important, fundamental-ish skills that project doesn't begin to touch on, though, especially for front-end devs: working with the DOM, and thinking about how to break down and solve real-world problems. Without those skills, someone who's grasped the basics via js-assessment might still struggle in a technical phone screen or interview, even for a junior position.
I recently put together a few exercises on CodePen recently for an aspiring front-end developer, and I thought I'd share them here. Each requires DOM manipulation, each requires solving a set of small but realistic problems, and each starts to hint at concepts for organizing an overall approach to a problem.

Number-guessing game
Tic-tac-toe
Word-guessing game

If others find them useful, maybe I'll make and share some more, and figure out how to be a bit more official about it. Let me know…