Mapping 2017 parallax code

parallax.js

let $back;
const BACKGROUND_ID = '#background', SCROLL_SELECTOR = 'main';
//hard-coded backgoround image size to start parallax asap
let sizes = {
  width: 1678, height: 2481
};

$(document).on('turbolinks:load', () => {
  $(scrollSelector).on('scroll', handleParallax);
  setSize();
  setBackPosition('translateY(0px)', true)
})
function setSize(){
  $back = $(BACKGROUND_ID);
  const imgSrc = $back.find('img').attr('src');
  const backImg = new Image();
  backImg.onload = (e) => {
    sizes = {
      width: $back.width(),
      height: $back.height(),
    }
    handleParallax(e, true);
  }
  backImg.src = imgSrc;
}
$(window).resize(setSize);
$(document).on('turbolinks:before-visit', () => setBackPosition('translateY(0px)', true));
$(window).on('scroll', handleParallax);

function handleParallax(e, animate = false){
  const $this = $(e.currentTarget);
  let height;
  if($this.is(SCROLL_SELECTOR)){
    const heights = $this.children().map((i,elem) => $(elem).outerHeight(true));
    height = Math.max(...heights);
  }
  else{
    height = $(scrollSelector).outerHeight(true);
  }
  // don't exagerate it...
  height = Math.max(height, 3*window.innerHeight);

  let percentageScrolled = ($this.scrollTop() / (height - window.innerHeight));
  percentageScrolled = Math.min(Math.max(0, percentageScrolled), 100);

  let transform = Math.ceil(percentageScrolled*(sizes.height - window.innerHeight));
  transform = `translateY(-${transform}px)`;

  window.requestAnimationFrame(() => setBackPosition(transform, animate));
}

function setBackPosition(transform, animate = false){
  const fn = animate ? 'transition' : 'css';
  $back[fn]({ transform })
}