ScrollSpy in pure javascript

scrollspy.js

/*
Copyright (C) 2021 Pascal de Vink (Tweakers.net)

This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
*/
var ScrollSpy = (function()
 {
    var elements = {};

    function init()
    {
        if (document.addEventListener)
        {
            document.addEventListener("touchmove", handleScroll, false);
            document.addEventListener("scroll", handleScroll, false);
        }
        else if (window.attachEvent)
        {
            window.attachEvent("onscroll", handleScroll);
        }
    }

    function spyOn(domElement)
    {
        var element = {};
        element['domElement'] = domElement;
        element['isInViewPort'] = true;
        elements[domElement.id] = element;
    }

    function handleScroll()
    {
        var currentViewPosition = document.documentElement.scrollTop ? document.documentElement.scrollTop: document.body.scrollTop;

        for (var i in elements) {
            var element = elements[i];
            var elementPosition = getPositionOfElement(element.domElement);

            var usableViewPosition = currentViewPosition;
            if (element.isInViewPort == false)
            {
                usableViewPosition -= element.domElement.clientHeight;
            }

            if (usableViewPosition > elementPosition)
            {
                fireOutOfSightEvent(element.domElement);
                element.isInViewPort = false;
            }
            else if (element.isInViewPort == false)
            {
                fireBackInSightEvent(element.domElement);
                element.isInViewPort = true;
            }
        };
    }

    function fireOutOfSightEvent(domElement)
    {
        fireEvent('ScrollSpyOutOfSight', domElement);
    }

    function fireBackInSightEvent(domElement)
    {
        fireEvent('ScrollSpyBackInSight', domElement);
    }

    function fireEvent(eventName, domElement)
    {
        if (document.createEvent)
        {
            var event = document.createEvent('HTMLEvents');
            event.initEvent(eventName, true, true);
            event.data = domElement;
            document.dispatchEvent(event);
        }
        else if (document.createEventObject)
        {
            var event = document.createEventObject();
            event.data = domElement
            event.expando = eventName;
            document.fireEvent('onpropertychange', event);
        }
    }

    function getPositionOfElement(domElement)
    {
        var pos = 0;
        while (domElement != null)
        {
            pos += domElement.offsetTop;
            domElement = domElement.offsetParent;
        }
        return pos;
    }

    return {
        init: init,
        spyOn: spyOn
    };
});

@bearbear12345 and others:

var scrollSpy = new ScrollSpy();
scrollSpy.init();

var el = document.querySelector('#nav');
scrollSpy.spyOn(el);

/* Element is visible again on the screen. */
document.addEventListener('ScrollSpyBackInSight', function (e) {
    /* ie. Make the nav to float. */
  el.style.position = 'relative';
});

/* Element is not visible anymore on the screen. */
document.addEventListener('ScrollSpyOutOfSight', function (e) {
  /* ie. Make the nav to fix. */
  el.style.position = 'fixed';
  el.style.top = '0px';
});

I can be wrong but that's how i'd use it.

And this is ES6 version:

const scrollSpy = new function () {
  const elements = {}

  const init = () => {
    if (document.addEventListener) {
      document.addEventListener('touchmove', handleScroll, false)
      document.addEventListener('scroll', handleScroll, false)
    }
    else if (window.attachEvent) {
      window.attachEvent('onscroll', handleScroll)
    }
  }

  const spyOn = domElement => {
    const element = {}
    element['domElement'] = domElement
    element['justGotIntoViewPort'] = true
    elements[domElement.id] = element
  }

  const handleScroll = () => {
    const currentViewPosition = document.documentElement.scrollTop
      ? document.documentElement.scrollTop
      : document.body.scrollTop

    for (let element of elements) {
      const elementPosition = getPositionOfElement(element.domElement)

      let usableViewPosition = currentViewPosition
      if (!element.isInViewPort) {
        usableViewPosition -= element.domElement.clientHeight
      }

      if (usableViewPosition > elementPosition) {
        fireOutOfSightEvent(element.domElement)
        element.isInViewPort = false
      } else if (!element.isInViewPort) {
        fireBackInSightEvent(element.domElement)
        element.isInViewPort = true
      }
    }
  }

  const fireOutOfSightEvent = domElement => {
    fireEvent('ScrollSpyOutOfSight', domElement)
  }

  const fireBackInSightEvent = domElement => {
    fireEvent('ScrollSpyBackInSight', domElement)
  }

  const fireEvent = (eventName, domElement) => {
    if (document.createEvent) {
      const event = document.createEvent('HTMLEvents')
      event.initEvent(eventName, true, true)
      event.data = domElement
      document.dispatchEvent(event)
    } else if (document.createEventObject) {
      const event = document.createEventObject()
      event.data = domElement
      event.expando = eventName
      document.fireEvent('onpropertychange', event)
    }
  }

  const getPositionOfElement = domElement => {
    let pos = 0
    while (domElement) {
      pos += domElement.offsetTop
      domElement = domElement.offsetParent
    }
    return pos
  }

  return {init, spyOn}
}()

scrollSpy.init()
export { scrollSpy }

usage

import { scrollSpy } from './ScrollSpy'
// // // // // // // // stuff
scrollSpy.spyOn(foo)
// // // // // then add listeners and whatever

Good job, man!