I've experienced a similar drawback as of late—wanting to contribute but not entirely sure of my skills. I think a viable solution for this would be to use JS meetups in your area as an opportunity to start your own open-source project. The group could be relatively small, and the bi-monthly or monthly meetups paired with a Slack channel could get the project off the ground easily. Your first missteps or issues could then be played out in a small, controlled environment.
As an aside, a useful resource I've found is the guidelines set forth on this Scotch.io tutorial by Noman Ur Rehman: scotch.io/tutorials/contributing-to-open-source-s…