const prevlink = document.getElementById('prev-link');
const nextlink = document.getElementById('next-link');
const posts = document.getElementsByTagName('article');

function keyUp(e) {
  var i, post;

  var el = document.activeElement;
  if (! (el && (el.tagName.toLowerCase() == 'input' || el.tagName.toLowerCase() == 'textarea'))) {
    if (! (e.ctrlKey || e.altKey || e.shiftKey || e.metaKey )) {
      if (e.code === 'ArrowLeft') {
        if (prevlink) {
          window.location.href = prevlink.href;
        } else if (posts) {
          i = posts.length-1;
          post = posts[i];
          while (i >= 0 && post.offsetTop + 5 > window.scrollY) {
            post=posts[i--];
          }
          post.scrollIntoView()
        }
      } else if (e.code === 'ArrowRight') {
        if (nextlink) {
          window.location.href = nextlink.href;
        } else if (posts) {
          i = 0;
          post = posts[i];
          while (i < posts.length && post.offsetTop - 5 < window.scrollY) {
            post=posts[i++];
          }
          post.scrollIntoView()
        }
      }
    }
  }
}

document.addEventListener('keyup', keyUp, false);