The original code is really nice:

  // golfed minslides, 173 bytes
  let a=document.getElementsByClassName("slide"),b=0,c=a.length-1;
  document.addEventListener("keypress",({key:d})=>{b+=("j"==d)-("k"==d),b=0>b?0:b>l?l:b,a[b].scrollIntoView()})