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const smoothScroll = () =>{
  let links = document.querySelectorAll('a[href^="#"]');
  const speed = 400;          // スクロールスピード
  const divisor = 100;        // 分割数
  const tolerance = 5;        // 許容誤差
  const headerHeight = 100;     // 固定ヘッダーがある場合はその高さ分ずらす
  const interval = speed/divisor;
  for(let i = 0; i < links.length; i++){
    links[i].addEventListener('click',(e)=>{
      e.preventDefault();
      let nowY = window.pageYOffset;
      const href = e.currentTarget.getAttribute('href');
      const target = document.querySelector(href);
      if( target != null){
        const targetRectTop = target.getBoundingClientRect().top;
        const targetY = targetRectTop + nowY - headerHeight;
        const minY = Math.abs((targetY - nowY)/divisor);
        doScroll(minY,nowY,targetY,tolerance,interval);
      }
    });
  }
}

const doScroll = (minY,nowY,targetY,tolerance,interval) =>{
  let toY ;
  if( targetY < nowY ){
    toY = nowY - minY;
  }else{
    toY = nowY + minY;
  }
  window.scrollTo(0, toY);
  if( targetY - tolerance > toY || toY > targetY + tolerance){
    window.setTimeout(doScroll,interval,minY,toY,targetY,tolerance,interval);
  }else{
    return false;
  }
}
smoothScroll();