page scroll js

page-scroll.js

  const body       = document.body;
  const scrollUp   = "scroll-up";
  const scrollDown = "scroll-down";
  let lastScroll   = 0;
  
  window.addEventListener("scroll", () => {
    const currentScroll = window.pageYOffset;
    if (currentScroll <= 0) {
      body.classList.remove(scrollUp);
      return;
    }
    
    if (currentScroll > lastScroll && !body.classList.contains(scrollDown)) {
      // down
      body.classList.remove(scrollUp);
      body.classList.add(scrollDown);
    } 
    else if (currentScroll < lastScroll && body.classList.contains(scrollDown)) {
      // up
      body.classList.remove(scrollDown);
      body.classList.add(scrollUp);
    }
    lastScroll = currentScroll;
  });