How to make a smooth dropdown menu in pure js?

Question

css

How to make a smoothly drop-down menu element with sub-items in pure JS or CSS?
So that when you click on the menu items, it would drop down smoothly.
Here is what I have
https://codepen.io/Shaltyr/pen/QWOeBXL
I also found what should be
https://codepen.io/fainder/pen/qBwBeO?editors=0110
But here on JQuery. And to me on pure JS.
Ideas were certainly, but so far unsuccessfully (((

Answer 1

https://github.com/nefe/You-Don't-Need-jQuery