Prism Line Number Plugin

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The Mozilla Developer Network (MDN) is in the midst of a remodel (sorry for the spoiler) and we've been implementing features incrementally.  One larger front-end change we'll be implementing is using the PrismJS for document code syntax highlighting.  One requirement of MDN's syntax highlighter is that line numbers are displayed, a functionality not provided by Prism.  Prism does provide a line-highlighting plugin but not a line-numbering plugin, so I used that plugin as my base and simply simplified it to provide numbers for a lines.


The CSS here is copied from the line-highlighting plugin.  I've changed the attribute to data-number, which we'll use instead of data-line, and I've changed the colors used for the line background:

pre[data-number] {
	position: relative;
	padding: 1em 0 1em 3em;

.line-number {
	position: absolute;
	left: 0;
	right: 0;
	padding: inherit 0;
	margin-top: 1em; /* Same as .prism's padding-top */

	background: transparent;
	pointer-events: none;
	line-height: inherit;
	white-space: pre;

	.line-number[data-end]:after {
		content: attr(data-start);
		position: absolute;
		top: .4em;
		left: .6em;
		min-width: 1em;
		padding: 0 .5em;
		color: #999;
		font: bold 65%/1.5 sans-serif;
		text-align: center;
		vertical-align: .3em;
		border-radius: 999px;
		text-shadow: none;
		border: 0;
	.line-number[data-end]:after {
		content: attr(data-end);
		top: auto;
		bottom: .4em;

Of course you can style the line number element to however you'd like, but this CSS keeps styles fairly consistent with the line-highlighting plugin.

The JavaScript

Here's the JavaScript portion for the plugin in all its glory:

	This plugins was created based on the Prism line-numbering plugin.
	This plugin aims to number all lines and is independent of highlighting.

if(!window.Prism || !document.querySelectorAll) {

function $$(expr, con) {
	return || document).querySelectorAll(expr));
function numberLines(pre) {
	var offset = +pre.getAttribute('data-line-offset') || 0;
	var lineHeight = parseFloat(getComputedStyle(pre).lineHeight);
	var code = pre.querySelector('code');
	var numLines = code.innerHTML.split('\n').length;
	pre.setAttribute('data-number', '');

	for (var i=1; i <= numLines; i++) {
		var line = document.createElement('div');
		line.className = 'line-number';
		line.setAttribute('data-start', i); = (i - offset - 1) * lineHeight + 'px';
		(code || pre).appendChild(line);

Prism.hooks.add('after-highlight', function(env) {
	var pre = env.element.parentNode;
	if (!pre || !/pre/i.test(pre.nodeName)) {

	$$('.line-number', pre).forEach(function (line) {


This code is also based on the syntax-highlighting plugin's code, but much more simplified because there's less line-numbering logic.

If there's enough interest in what I have, I can make a GitHub repo for the plugin.  I haven't to this point because I believe this plugin could/should be merged with the line-highlighting plugin, since in many cases, developers reference line numbers.  In any event, this plugin may be what you're looking for.  Let me know if you have updates or ideas!

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  1. Jonas

    This fails in WebKit, as getComputedStyle(pre).lineHeight returns ‘normal’.
    See also:

    • Ahh yes Jonas; I encountered that issue and explicitly set a line-height within a different stylesheet so as to avoid this issue.

  2. Maybe you missed it, but PrismJS does have a line numbering plugin. More info can be found on

    • Hmm, that may be new. Thanks Mark!

    • So I just took a look at this and the line-numbers plugin isn’t compatible with the line-highlight plugin; mine is.

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