SOURCE

// scrollHeight滚动高度
function scrollToTop (y = 0, n = 8) {
  if (typeof y !== 'number' && !isNaN(y)) {
    return
  }
  var raf = window.requestAnimationFrame ||
            window.webkitRequestAnimationFrame ||
            window.mozRequestAnimationFrame ||
            window.msRequestAnimationFrame ||
            window.oRequestAnimationFrame ||
            (cb => setTimeout(cb, 1000 / 60))
  var cRaf = window.cancelAnimationFrame ||
          window.webkitCancelAnimationFrame ||
          window.webkitCancelRequestAnimationFrame ||
          window.mozCancelRequestAnimationFrame ||
          window.oCancelRequestAnimationFrame ||
          window.msCancelRequestAnimationFrame ||
          clearTimeout
  var timer = null
  var loop = () => {
    var c = (document.documentElement.scrollTop || document.body.scrollTop) - y
    if (Math.abs(c / n) < 1) {
      scrollTo(0, y)
      cRaf(timer)
      timer = null
      return
    }
    timer = raf(loop)
    scrollTo(0, (document.documentElement.scrollTop || document.body.scrollTop) - c / n)
    // setInterval计时器版本
    // timer = setInterval(() => {
    //   var c = (document.documentElement.scrollTop || document.body.scrollTop) - y
    //   if (Math.abs(c / n) < 1) {
    //     scrollTo(0, y)
    //     cRaf(timer)
    //     timer = null
    //     return
    //   }
    //   scrollTo(0, (document.documentElement.scrollTop || document.body.scrollTop) - c / n)
    // }, 1000 / 60)
  }
  return loop()
}


function scrollToElement (el, n = 8) {
  var y = el.offsetTop
  var raf = window.requestAnimationFrame ||
            window.webkitRequestAnimationFrame ||
            window.mozRequestAnimationFrame ||
            window.msRequestAnimationFrame ||
            window.oRequestAnimationFrame ||
            (cb => window.setTimeout(cb, 1000 / 60))
  var cRaf = window.cancelAnimationFrame ||
          window.webkitCancelAnimationFrame ||
          window.webkitCancelRequestAnimationFrame ||
          window.mozCancelRequestAnimationFrame ||
          window.oCancelRequestAnimationFrame ||
          window.msCancelRequestAnimationFrame ||
          window.clearTimeout
  var timer = null
  var loop = () => {
    var c = (document.documentElement.scrollTop || document.body.scrollTop) - y
    if (Math.abs(c / n) < 1) {
      scrollTo(0, y)
      cRaf(timer)
      timer = null
      return
    }
    timer = raf(loop)
    scrollTo(0, (document.documentElement.scrollTop || document.body.scrollTop) - c / n)
    // setInterval计时器版本
    // timer = setInterval(() => {
    //   var c = (document.documentElement.scrollTop || document.body.scrollTop) - y
    //   if (Math.abs(c / n) < 1) {
    //     scrollTo(0, y)
    //     cRaf(timer)
    //     timer = null
    //     return
    //   }
    //   scrollTo(0, (document.documentElement.scrollTop || document.body.scrollTop) - c / n)
    // }, 1000 / 60)
  }
  return loop()
}