How to scroll fix an element

scroll-fixed.js

// The following code is all pseudo-code

let elem = ...; // get element we want to fix
let initScrollPosition = ...; // get initial position of element

// the following pseudo-code should be is in a scrolling listener
if (window.scrollYLevel > initScrollPosition) {
  elem.css.position = 'fixed';
  elem.css.top = 0; // 0 or other position we'd want
} else {
  elem.css.position = 'static';
  elem.css.top = 'auto'; // return back to the initial position
}