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. Then, I thought I want to make it a little bit fancy by adding the line numbers into it. Finally, I realized it by using few lines of JavaScript code and a little bit of styles using CSS code. Let’s the code on how I implemented it for this site (a Jekyll powered site) without using any code syntax-highlighting plugin.

Since the markdown will output the code block into something like <pre><code>...</code></pre>, so I can use the JavaScript vanilla 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 code above and include it before the </body> tag of your Jekyll layout.

Below is my SCSS code for styling the code block layout as you can see on this page 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 did for this Jekyll site. Have fun coding!