How should I force an element to remain fixed with parallax scrolling?

Abridged Codepen below. I’m trying to model a page after this BBC article from way back, but I can’t seem to get my image to go to position fixed based off scrolling.

I’m thinking that if I can get the element to be position: fixed then I can swap out the images with js

I can’t seem to get my yPosition var to pass through my scroll function though

Here’s the fuller codepen of the project, I just can’t seem to get this js to work for me. Any tips?

here’s my JS

var yPosition = document.getElementById("scrollLock").offsetTop;

window.onscroll = function(){ // scrolling fires this function      
    var myElement = document.getElementById("scrollLock"); // for cleaner code

    // compare original y position of element to y position of page
  console.log('it is scrolled at');
    if( yPosition < window.pageYOffset ){ 

        // snap element to the top by changing css values of element
        console.log('it worked');
     } else {          

        // re-position to original flow of content
       console.log('it passed');

addClass() is not a JS function. You can use classList for this: