highlight.js

What is highlight.js?

The highlight.js npm package is a syntax highlighter written in JavaScript. It's used to add syntax highlighting to code blocks on web pages, making them more readable and aesthetically pleasing. It supports a wide range of programming languages and is commonly used in blogs, forums, and other platforms where code is shared.

What are highlight.js's main functionalities?

Syntax Highlighting

Automatically detects and highlights syntax in code blocks on a webpage. This is the most basic usage where it applies highlighting to all code blocks.

hljs.highlightAll();

Custom Language Selection

Highlights a specific code element with a specified language. This allows for more control over which elements are highlighted and in what language.

hljs.highlightElement(document.getElementById('my-code'), {language: 'javascript', ignoreIllegals: true});

Custom Themes

Allows the use of custom themes for syntax highlighting. Themes are available as separate CSS files that can be imported to change the appearance of highlighted code.

import 'highlight.js/styles/atom-one-dark.css';

Line Numbers

Adds line numbers to code blocks. This feature is often used in conjunction with syntax highlighting to improve readability and reference specific lines of code.

document.addEventListener('DOMContentLoaded', (event) => { document.querySelectorAll('pre code').forEach((block) => { hljs.lineNumbersBlock(block); }); });

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README

Highlight.js

Highlight.js is a syntax highlighter written in JavaScript. It works in the browser as well as on the server. It works with pretty much any markup, doesn’t depend on any framework, and has automatic language detection.

Upgrading from Version 9

Version 10 is one of the biggest releases in quite some time. If you're upgrading from version 9, there are some breaking changes and things you may want to double check first.

Please read VERSION_10_UPGRADE.md for high-level summary of breaking changes and any actions you may need to take. See VERSION_10_BREAKING_CHANGES.md for a more detailed list and CHANGES.md to learn what else is new.

Getting Started

The bare minimum for using highlight.js on a web page is linking to the library along with one of the styles and calling initHighlightingOnLoad:

<link rel="stylesheet" href="/path/to/styles/default.css">
<script src="/path/to/highlight.min.js"></script>
<script>hljs.initHighlightingOnLoad();</script>

This will find and highlight code inside of <pre><code> tags; it tries to detect the language automatically. If automatic detection doesn’t work for you, you can specify the language in the class attribute:

<pre><code class="html">...</code></pre>

Classes may also be prefixed with either language- or lang-.

<pre><code class="language-html">...</code></pre>

Plaintext and Disabling Highlighting

To style arbitrary text like code, but without any highlighting, use the plaintext class:

<pre><code class="plaintext">...</code></pre>

To disable highlighting of a tag completely, use the nohighlight class:

<pre><code class="nohighlight">...</code></pre>

Supported Languages

Highlight.js supports over 180 different languages in the core library. There are also 3rd party language plugins available for additional languages. You can find the full list of supported languages in SUPPORTED_LANGUAGES.md.

Custom Initialization

When you need a bit more control over the initialization of highlight.js, you can use the highlightBlock and configure functions. This allows you to control what to highlight and when.

Here’s an equivalent way to calling initHighlightingOnLoad using vanilla JS:

document.addEventListener('DOMContentLoaded', (event) => {
  document.querySelectorAll('pre code').forEach((block) => {
    hljs.highlightBlock(block);
  });
});

You can use any tags instead of <pre><code> to mark up your code. If you don't use a container that preserves line breaks you will need to configure highlight.js to use the <br> tag:

hljs.configure({useBR: true});

document.querySelectorAll('div.code').forEach((block) => {
  hljs.highlightBlock(block);
});

For other options refer to the documentation for configure.

Web Workers

You can run highlighting inside a web worker to avoid freezing the browser window while dealing with very big chunks of code.

In your main script:

addEventListener('load', () => {
  const code = document.querySelector('#code');
  const worker = new Worker('worker.js');
  worker.onmessage = (event) => { code.innerHTML = event.data; }
  worker.postMessage(code.textContent);
});

In worker.js:

onmessage = (event) => {
  importScripts('<path>/highlight.min.js');
  const result = self.hljs.highlightAuto(event.data);
  postMessage(result.value);
};

Node.js

You can use highlight.js with node to highlight content before sending it to the browser. Make sure to use the .value property to get the formatted html. For more info about the returned object refer to the api docs https://highlightjs.readthedocs.io/en/latest/api.html

// require the highlight.js library including all languages
const hljs = require('./highlight.js');
const highlightedCode = hljs.highlightAuto('<span>Hello World!</span>').value

// require the highlight.js library without languages
const hljs = require("highlight.js/lib/core");
// separately require languages
hljs.registerLanguage('html', require('highlight.js/lib/languages/html'));
hljs.registerLanguage('sql', require('highlight.js/lib/languages/sql'));
// highlight with providing the language
const highlightedCode = hljs.highlight('html', '<span>Hello World!</span>').value

Getting the Library

You can get highlight.js as a hosted, or custom-build, browser script or as a server module. Right out of the box the browser script supports both AMD and CommonJS, so if you wish you can use RequireJS or Browserify without having to build from source. The server module also works perfectly fine with Browserify, but there is the option to use a build specific to browsers rather than something meant for a server. Head over to the download page for all the options.

Don't link to GitHub directly. The library is not supposed to work straight from the source, it requires building. If none of the pre-packaged options work for you refer to the building documentation.

The CDN-hosted package doesn't have all the languages. Otherwise it'd be too big. If you don't see the language you need in the "Common" section, it can be added manually:

<script
 charset="UTF-8"
 src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/9.15.9/languages/go.min.js"></script>

On Almond. You need to use the optimizer to give the module a name. For example:

r.js -o name=hljs paths.hljs=/path/to/highlight out=highlight.js

CommonJS

You can import Highlight.js as a CommonJS-module:

npm install highlight.js --save

In your application:

import hljs from 'highlight.js';

The default import imports all languages! Therefore it is likely to be more efficient to import only the library and the languages you need:

import hljs from 'highlight.js/lib/core';
import javascript from 'highlight.js/lib/languages/javascript';
hljs.registerLanguage('javascript', javascript);

To set the syntax highlighting style, if your build tool processes CSS from your JavaScript entry point, you can import the stylesheet directly into your CommonJS-module:

import hljs from 'highlight.js/lib/core';
import 'highlight.js/styles/github.css';

License

Highlight.js is released under the BSD License. See LICENSE file for details.

Links

The official site for the library is at https://highlightjs.org/.

Further in-depth documentation for the API and other topics is at http://highlightjs.readthedocs.io/.

Authors and contributors are listed in the AUTHORS.txt file.