How to properly organize the logic of the slider module?

Question

JavaScript

Good afternoon,
I needed to make a slider, like here: artgorbunov.ru
I present some moments separately.
But there is no general picture.
I went into their code, it is written a little chaotically, but it needs to be done quickly, there is very little time to figure it out, some points are clear - everything as a whole is a mess, and in (mine) code too). At least an approximate algorithm.
And how to correctly calculate the percentages between a small slider and a large one. I am very dumb.
Help, please, if someone can!

Answer 1

In one of our projects, we used the Sly library . Everything you need is there. But you will have to finish it with a file yourself =)