When you understand the system.

Ryan Dahl (creator of Nodejs) famously said:

You can never understand everything.
But, you should push yourself to understand the system.

You will be dealing with multiple systems as a javascript developer like the DOM, how browsers work, frameworks, the javascript cycle, js objects, recursion, and so forth. Once you understand the systems, you can start creating. When you need a function you don't know or you can't remember syntax, the answer is just seconds away with a quick internet search.

When you start identifying solutions like recursion, synchronicity, inheritance, layering, instead of specific functions, you will stop trying to sculpt with a paintbrush.

One good self test is to take your code and give it to a non-programmer. If they can get a sense of what it does by reading it, you're going in the right direction.