Adding line numbers to the code block layout for my Jekyll site

Previously, my code block for this site is just a simply boring code block. So, I thought I want to add the line numbers for my code block and I did it using few lines of JavaScript and then styled it nicely using CSS code. I thought I want to share it here, so what you will see on my code block here later, that’s how I implemented it for my Jekyll theme (this site) without using other code syntax-highlighting plugin whatsoever.

Since the markdown will output the code block into something like <pre><code>...</code></pre>, so I can use pure JavaScript to create the line numbers and inject the HTML code into my <pre><code> block.

(function() {
  var pre = document.getElementsByTagName('pre'),
      pl = pre.length;
  for (var i = 0; i < pl; i++) {
    pre[i].innerHTML = '<span class="line-number"></span>' + pre[i].innerHTML + '<span class="cl"></span>';
    var num = pre[i].innerHTML.split(/\n/).length;
    for (var j = 0; j < (num - 1); j++) {
      var line_num = pre[i].getElementsByTagName('span')[0];
      line_num.innerHTML += '<span>' + (j + 1) + '</span>';

If you want to implement into your site, you can use the JavaScript above and include it before the </body> tag of your Jekyll layout.

Below is my SCSS code for styling my code block layout as you can see on this article right now:

pre {
  display: block;
  margin-top: 0;
  margin-bottom: 1rem;
  font-size: 0.7rem;
  line-height: 1.4;
  white-space: pre;
  overflow: auto;
  background-color: #f9f9f9;
  border: 1px solid #ddd;
  padding: .5rem;
  max-height: 800px;
  font-family: monospace;

  code {
    color: inherit;
    background-color: transparent;
    padding: 0;
    display: block;

  .line-number {
    display: block;
    float: left;
    margin: 0 1em 0 -1em;
    border-right: 1px solid #ddd;
    text-align: right;

    span {
      display: block;
      padding: 0 .5em 0 1em;
      color: #ccc;

  .cl {
    display: block;
    clear: both;

If you want to try and play around with the demo, you can check my CodePen here, but please note that the markdown in the CodePen does not have automatic code syntax-highlighting like GitHub Pages does for this Jekyll site. Have fun coding!