* {
  box-sizing: border-box;
}
.header {
  width: 100%;
  height: 70px;
  background: #000;
  color: #fff;
  position: fixed;
  top: 0;
  left: 0;
  display:flex;
  align-items:center;
  justify-content:center;
  transition: .2s;
}
.header.hide {
  top:-70px;
}
section {
  border: 5px solid gray;
  height: 500px;
  background: #47FF7AE5;
  display:flex;
  align-items:center;
  justify-content:center;
}

.goTop {
  position: fixed;
  right: 20px;
  bottom: 20px;
  padding: 40px;
  border-radius: 50px;
  background: #fff;
  opacity: 0;
  visibility: hidden;
  transition: .2s;
}

.goTop.show{
  opacity: 1;
  visibility: visible;
}

const header = document.querySelector(".header")

const goTop = document.querySelector(".goTop")

function scrollEvent(event){
  const STANDARD = 30;
  
  if(window.scrollY > STANDARD){
    header.classList.add("hide")
    goTop.classList.add("show")
  }else {
    header.classList.remove("hide")
    goTop.classList.remove("show")
  }
  
}

function moveTop(){
    window.scrollTo({
      top: 0,
      left: 0,
      behavior: 'smooth'
    });
}

window.addEventListener("scroll", scrollEvent)
goTop.addEventListener("click", moveTop)