goTop () {
   cancelAnimationFrame(this.timer)
   const self = this
   self.timer = requestAnimationFrame(function fn () {
   const oTop = document.body.scrollTop || document.documentElement.scrollTop
   if (oTop > 0) {
      document.body.scrollTop = document.documentElement.scrollTop = oTop - 50
      self.timer = requestAnimationFrame(fn)
    } else {
      cancelAnimationFrame(self.timer)
    }
  })
}