Script to add class on scroll?

Question

css

What is the script for this task:
There are 4 blocks on the page, one under the other.
it is necessary if I am on block 1, then add the .active class.
When I scroll to another, it is removed there and added to the second.

Answer 1

jsfiddle.net/mekwall/up4nu

Answer 2

https://jsfiddle.net/grigruss/byhst41L/