1 - Codility is a great way to get started on interview-style coding problems. They have lessons (unfortunately in python) to demonstrate a computer science principle (e.g. stacks and queues), then a few problems to practice this principle. You can solve the problems in any language and you can easily google other people's solutions if you get stuck.
2 - A second great resource is interviewCake. It's a paid resource but there are coupons all over the web, and you can sign up for a free question once a week.
3 - For a beginner's introduction to JS problems, codewars is a pretty smooth introduction.
4 - Another beginner's intro to JS problem solving is the basic algorithm section on freeCodeCamp. You can't look at other people's submissions, but again, if you google the title of the problem you'll find lots of github pages with decent answers.
5 - Finally, I use HackerRank. There's an initial hurdle of getting over the way they give you input (you have to parse through data), but it's pretty good once you get the hang of it.
Good Luck!