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HTML

              
                <div>
  <h1>Smooth Scroll JavaScript (Easing support)</h1>

  <p><a href="#img">鹿まで移動</a></p>

  <p><img src="https://picsum.photos/id/275/400/500" alt="" width="400" height="500"></p>
  <p><img src="https://picsum.photos/id/200/800/500" alt="" width="800" height="500"></p>
  <p><img src="https://picsum.photos/id/1062/1600/500" alt="" width="1600" height="500"></p>

  <p id="img"><img src="https://picsum.photos/id/1003/500/600" alt="" width="500" height="600"></p>

  <p><img src="https://picsum.photos/id/428/1600/700" alt="" width="1600" height="700"></p>

  <a href="#top">ページTOPへ</a>
</div>
              
            
!

CSS

              
                div {
  margin: 0;
  padding: 100px 0;
  text-align: center;
}
h1 {
  text-align: center;
  margin-bottom: 10vh;
}
p {
  margin: 100px 0;
}
a {
  font-size: 30px;
}
img {
  max-width: 100%;
  vertical-align: top;
}
#img {
  scroll-margin-top: 40px;
  scroll-padding-top: 3rem;
}
              
            
!

JS

              
                /**
 * スムーススクロール関数
 */
const smoothScroll = () => {
	//スクロールリンク取得
	const targets = document.querySelectorAll('a[href^="#"]');
	if(targets.length === 0) {
		return;
	}
  
	//スクロール速度
	const ANIME_SPEED = 600;

	/*
		イージング設定
		https://easings.net/ja
	*/
	function easing(x) {
		//easeInOutQuad
    return x < 0.5 ? 2 * x * x : 1 - Math.pow(-2 * x + 2, 2) / 2;
  }
  
  //移動先の位置を取得する関数
  function getScrollStopTop(scrollStopTarget) {
    //scroll-margin-top、scroll-padding-topを取得。相対値による小数点は四捨五入
    const scrollStopTargetStyles = getComputedStyle(scrollStopTarget);
    const scrollPaddingTop = scrollStopTargetStyles.scrollPaddingTop;
    const shiftPosition = Math.round(parseFloat(scrollStopTargetStyles.scrollMarginTop)) + Math.round(parseFloat(scrollPaddingTop === 'auto' ? 0 : scrollPaddingTop));
    return Math.round(scrollStopTarget.getBoundingClientRect().top) - shiftPosition;
  }

	const scrollBusyClass = 'is-scroll-busy';
	const bodyClassList = document.body.classList;
  
	//クリックイベント設定
	targets.forEach(target => {
		target.addEventListener('click', event => {
			event.preventDefault();

			//スクロールイベント重複防止
			if (bodyClassList.contains(scrollBusyClass)) {
				return;
			}
			bodyClassList.add(scrollBusyClass);

			//hrefから遷移先を取得
			const href = target.getAttribute('href');
      let scrollStopTarget = null;
			let selector = href;
			//#top(HTMLの仕様でページの最上部へ戻るアンカーリンク)が手動設定されているかチェック
			//https://html.spec.whatwg.org/multipage/browsing-the-web.html#scroll-to-the-fragment-identifier
      if(href === '#top') {
        scrollStopTarget = document.querySelector(href);
      }
			//#topが手動設定されていない場合はhrefから遷移先を取得
      if(!scrollStopTarget) {
				//#や#topの場合はページの最上部へ戻るbodyを遷移先に設定
				selector = (href === '#' || href === '#top') || href === '' ? 'body' : href;
        scrollStopTarget = document.querySelector(selector);
      }

			//遷移先が存在しない場合は処理しない
			if (!scrollStopTarget) {
				bodyClassList.remove(scrollBusyClass);
				return;
			}

			//現在のスクロール量
			const scrollTop = window.scrollY;

      //移動先取得
			let scrollStopTop = 0;
			if(selector === 'html') {
				scrollStopTop = -scrollTop;
			} else {
				scrollStopTop = getScrollStopTop(scrollStopTarget);
			}


			//アニメーション開始時間
			let start;
			//スクロールアニメーション関数
			const startAnime = () => {
				requestAnimationFrame(mainAnime);
			}
			const mainAnime = timestamp => {

				//イベント発生後の経過時間
				// start未定義時のみtimestampを代入することで一度だけstartに数値を格納する
				if (start === undefined) {
					start = timestamp;
				}

				// 経過時間を監視
				const elapsedTime = timestamp - start;

				//アニメーション終了処理
				if (elapsedTime > ANIME_SPEED) {
					//実行中class削除
					bodyClassList.remove(scrollBusyClass);

					//処理を終了
					return;
				}

				//進捗度を算出してイージングを適用
				//elapsedTimeを0からanimeSpeedの値で正規化(0〜1の値に変換)
				const elapsedTimeNorm = elapsedTime / ANIME_SPEED;

				//位置設定
				const scrollY = scrollTop + scrollStopTop * easing(elapsedTimeNorm);

				//スクロール処理
				window.scrollTo(
					0,
					scrollY
				);
				startAnime();
			}

			//アニメーション初回呼び出し
			startAnime();
      
      //クリックしたアンカーリンクをURLの履歴に追加する
      if(selector !== 'html') {
        history.pushState(null, null, href);
      }
		});
	});
}

//スムーススクロール関数呼び出し
smoothScroll();
              
            
!
999px

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