JavaScript ページのトップへスクトールさせるボタン

script.js

const PageTopBtn = document.getElementById('js-scroll-top');
if (PageTopBtn) {
  PageTopBtn.addEventListener('click', () => {
    window.scrollTo({
      top: 0,
      behavior: 'smooth'
    });
  });
}

index.html

<div id="js-scroll-top" class="scroll-top"></div>

style.css

.scroll-top {
  display: block;
  position: fixed;
  right: 40px;
  bottom: 50px;
  width: 50px;
  height: 50px;
  border-radius: 25px;
  color: #fff;
  background-color: gray;
  z-index: 500;
  text-align: center;
  vertical-align: bottom;
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
  box-shadow: 0 0 6px 0 rgba(0, 0, 0, 0.5);
  transition: 0.5s;
}
.scroll-top:hover {
  box-shadow: 0 4px 7px 0 rgba(0, 0, 0, 0.5);
  transform: translateY(-5px);
  cursor: pointer;
  opacity: 0.7;
}
.scroll-top:before {
display: block;
margin: -13px auto auto;
content: "";
position: absolute;
top: 50%;
right: 0;
left: 0;
width: 10px;
height: 10px;
z-index: 1;
border-top: solid 3px #ffffff;
border-right: solid 3px #ffffff;
-webkit-transform: rotate(-45deg);
transform: rotate(-45deg);
}
.scroll-top::after {
  display: block;
  margin: 25px auto auto;
  content: "TOP";
  font-size: 13px;
  line-height: 17px;
}