Not everyone that tries this transition succeeds, but many do. Here's a playbook for setting some milestones that are more granular than "develop a new feature".

• Participate here and start answering questions. People with your background are often really good at debugging.
• File GitHub issues for big reports here. Watch the project for duplicate reports and link them to the first report so devs can easily close dupes.
• File new bug reports for issues you find. When you do this, read the code to see if you can find the file, function, or line of code where the problem resides. Experienced programmers often spend 90% of their time looking for the problem and 10% fixing it. If you can locate the fault and describe it, you've often finished the hard part.
• Checkout the GitHub repo. Try to run the test suite. Try to build the app. Sideload your own build onto your phone for testing.
• If the JavaScript language is still looking intimidating to you, crank through something like https://www.w3schools.com/js/js_examples.asp , doing bits of every section at least up to functions or events.

At this point, writing small features will feel natural. You don't have to do everything in this list step by step, jump in wherever feels natural. But this is my zero to hero playbook when I want to get involved in a project that uses an unfamiliar language or otherwise feels intimidating to just drop a pull request into. Especially if you write software in some unpopular language, picking up a new language isn't out of reach. How long it takes is up to you, it might be a fevered night of intense study, a week of focused spare time effort, or months of unstructured messing around. Those are all valid paths.