One approach that's worked well for me is by doing video courses. There are different ways you can tackle this, but essentially you can create a few JavaScript courses and continuously sell them throughout the year or until they become outdated.
Websites like Teachable can help you host your own courses. Self hosting is a little more work but you keep what you make. training.talkpython.fm is a good example of a self hosted training site. There are also places like egghead.io, Pluralsight, Lynda.com, etc that will do the hosting and marketing for you. With those sites, you'll end up getting a percentage based on their respective formula.
So for me, I'll do my regular work during the day and record video courses at night at home when everyone is sleeping and I can get some quiet. I had to purchase a microphone and some recording software (e.g Camtasia), but that was all of my out of pocket expenses.
Cecil Phillip
.NET Software Developer, Podcaster, Educator & Mentor