How to make paint fill with JS+SVG?

Question

JavaScript

codepen.io/nikfakel/pen/ZWjEbW
Click on the word Pencil and try to draw a thread.
Here, between the resulting lines, it is necessary for the filling to work. The code is not needed, just tell me which way to move, which method to use, how can this be implemented at all? And is it even possible?