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                <header class="header">

<div class="for-scroll"></div>

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


  background: #ddd;
  color: #fff;
  width: 100%;
  height: 90px;
  text-align: center;

.header h1{
  font-size: 32px;
  font-weight: bold;
  line-height: 90px;

  height: 1500px;

.scroll-top {
  position: fixed;
  right: 25px;
  bottom: 25px;
  z-index: 100;
  background-color: #ddd;
  width: 50px;
  height: 50px;
  border-radius: 50%;
  color: #fff;
  line-height: 50px;
  text-align: center;
.scroll-top:hover {
  cursor: pointer;
  opacity: 0.7;

  opacity: 0;
  transition: opacity 350ms ease;

.is-hide {
  pointer-events: none;


                const pageTopBtn = document.getElementById('js-scroll-top');

window.addEventListener("scroll", () => {
  const currentY = window.pageYOffset;
  if ( currentY > 100){
    setTimeout(function(){ = 1;
    }, 1);
  } else {
    setTimeout(function(){ = 0;
    }, 1);

// スクロールトップボタン
scrollTop('js-scroll-top', 150); // 遅すぎるとガクガクになるので注意

function scrollTop(el, duration) {
  const target = document.getElementById(el);
  target.addEventListener('click', function() {
    let currentY = window.pageYOffset; // 現在のスクロール位置を取得
    let step = duration/currentY > 1 ? 10 : 100; // 三項演算子
    let timeStep = duration/currentY * step; // スクロール時間
    let intervalId = setInterval(scrollUp, timeStep);
    // timeStepの間隔でscrollUpを繰り返す。
    // clearItervalのために返り値intervalIdを定義する。

    function scrollUp(){
      currentY = window.pageYOffset;
      if(currentY === 0) {
          clearInterval(intervalId); // ページ最上部に来たら終了
      } else {
          scrollBy( 0, -step ); // step分上へスクロール