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<a href="#hello">クリック</a>

<div id="hello">Hello!</div>



                header {
  background-color: #000;
  height: 50px;
  text-align: center;
  color: #fff;
  font-size: 20px;
  display: flex;
  justify-content: center;
  align-items: center;

a {
  display: inline-block;
  margin-top: 40px;

#hello {
  text-align: center;
  margin-top: 500px;
  font-size: 30px;
  padding-bottom: 600px;


                  const offset = 50; // ヘッダーの高さを入れる
  const links = document.querySelectorAll('a[href^="#"]');//すべてのaタグ href属性に#が入ってるもの
// DOM内のa[href^="#"]の数だけループ
  links.forEach((link) => {
// a[href^="#"]をクリックしたら
    link.addEventListener('click', (event) => {
      let href = link.getAttribute('href').substr(1);      
      href = href == '#' ? 'html' : `[id="${href}"]`;
      const target = document.querySelector(href);
      const rect = target.getBoundingClientRect();
      const scrollTop = window.pageYOffset || document.documentElement.scrollTop;
      const position = + scrollTop - offset;
        top: position,
        behavior: 'smooth'