1) Start interviewing for something better immediately. It might be a while before something catches, but for many people, interviewing is a skill that doesn't come naturally and can only be improved by practice. Your salary is ridiculously low; there should be no reason for you to have any loyalty to your current employer.
2) There's still plenty to learn in an awful, underpaid job. First off, of course, practicing and honing your skills. Writing the best code you can within real-world time constraints. Reading other people's code and understanding a legacy code base.
Then there's logistical and interpersonal stuff. Working on a real-world team, and how it uses an issue tracker, version control, etc. Getting code review and learning to give good code review. Office politics.
In an office that's underpaying you that much, it's quite likely that a lot of things are insanely dysfunctional. That's OK! All offices are dysfunctional in one way or another. Learning to work around it is a good thing.
Just make sure that whatever the environment is around you, and as difficult as the people are to get along with, you're still being excellent to them. And try not to pick up really bad technical habits -- when you move on to the next thing, be as open as you can to learning better ways of doing things.
3) Go to tech meetups and network. You want to meet both senior engineers and people in engineering management positions. The former are great for networking and mentorship (and often refer people they know to others who are hiring); the latter are constantly hiring, especially those whose companies are in expansion modes.
4) Ask your senior engineering mentors to do mock interviews with you and help you understand where you might be running into problems with both technical and non-technical rounds.
5) Research which startups raised a major funding round 1-12 months ago. They're usually the ones in expansion mode. They're typically looking for oodles of junior engineers to place under the leadership of a smaller number of senior engineers.
6) Find yourself a recruiter. They've got clients desperately begging for javascript engineers (as I can attest, having recently been in the market to hire a front-end engineer), and it's in their interest to get you the highest-paying job you're anywhere near qualified for. The better ones may also be willing to help you practice non-technical interviews and offer feedback. Because it makes it easier for them to sell a client on you.
Yes, some recruiters are the scum of the earth, but there are also a lot of decent ones and a handful of great ones. If you've got a bit of skill and a non-toxic personality, even the halfway decent ones will find you a much better deal than you're currently getting.
(Just make sure that it's also a fit for you - obvious you want a salary around market rate, but don't let the recruiter shoehorn you into a high-paying position where you'll be miserable because of culture or other reasons.)
7) Practice negotiating!