Coding Challenges for Javascript Developers

Here is the list of competitive programming sites that allow you to use JavaScript in order to submit your solution:

1. HackerRank supports a lot of languages, has many active registered users (I believe something around 70-80k), smooth user experience and UI. Supports contests, splits problems in domains, so you can practice what you want and need. Recommended for JavaScript developers since you don't have a feeling of using old-school tools.
2. CodeEval has clunky UI/UX, not as comfortable as HackerRank and may confuse you with status of your submission. Apart from that it's good enough to give it a shot. The community is not that big, something around 10k I guess.
3. CodeWars looks nice and feels nice. It's focused more on sharpening language skills, so not too many languages are supported now, but you still have a lot of interesting problems to solve. Interesting ranking system and you can compare your solution with others.
4. SPOJ is also a well known place to practice your skills as well as participate and create in different contests. Haven't used too much, but looks good. A lot of people recommend it.
5. CoderByte. Personally I haven't used it, but it exists. At least this fact is worth of mentioning it :) Has not so many challenges.
6. The Project Euler only requires you to enter the right answer, therefore there is no restriction on the language you use. Incredibly popular as far as I know.

I want to keep this list up-to-date, so if you know more online judging systems and/or coding challenging platforms that support JavaScript, please ping me on Twitter.