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Closes GH-24.

Reviewed-by: Remco Haszing <remcohaszing@gmail.com>
Reviewed-by: Titus Wormer <tituswormer@gmail.com>
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rehype-highlight

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rehype plugin to apply syntax highlighting to code with highlight.js (through lowlight).

Contents

What is this?

This package is a unified (rehype) plugin to apply syntax highlighting to code with highlight.js. highlight.js is pretty fast, relatively small, and a quite good syntax highlighter which has support for up to 190 different languages. This package bundles 35 common languages by default and you can register more.

It looks for <code> elements (when directly in <pre> elements) and changes them. You can specify the code language (such as Python) with a language-* or lang-* class, where the * can be for example js (so language-js), md, css, etc. By default, code without such a language class is not highlighted. Pass detect: true to detect their programming language and highlight the code anyway. You can still prevent specific blocks from being highlighted with a no-highlight or nohighlight class on the <code>.

unified is a project that transforms content with abstract syntax trees (ASTs). rehype adds support for HTML to unified. hast is the HTML AST that rehype uses. This is a rehype plugin that applies syntax highlighting to the AST.

When should I use this?

This project is useful when you want to apply syntax highlighting in rehype. One reason to do that is that it typically means the highlighting happens once at build time instead of every time at run time.

There are several other community plugins that apply syntax highlighting. Some of them are great choices but some are broken. As anyone can make rehype plugins, make sure to carefully assess the quality of rehype plugins.

This plugin is built on lowlight, which is a virtual version of highlight.js. You can make a plugin based on this one with lowlight when you want to do things differently.

Install

This package is ESM only. In Node.js (version 12.20+, 14.14+, or 16.0+), install with npm:

npm install rehype-highlight

In Deno with esm.sh:

import rehypeHighlight from 'https://esm.sh/rehype-highlight@5'

In browsers with esm.sh:

<script type="module">
  import rehypeHighlight from 'https://esm.sh/rehype-highlight@5?bundle'
</script>

Use

Say we have the following file example.html:

<h1>Hello World!</h1>

<pre><code class="language-js">var name = "World";
console.warn("Hello, " + name + "!")</code></pre>

And our module example.js looks as follows:

import {read} from 'to-vfile'
import {rehype} from 'rehype'
import rehypeHighlight from 'rehype-highlight'

const file = await rehype()
  .data('settings', {fragment: true})
  .use(rehypeHighlight)
  .process(await read('example.html'))

console.log(String(file))

Now running node example.js yields:

<h1>Hello World!</h1>

<pre><code class="hljs language-js"><span class="hljs-keyword">var</span> name = <span class="hljs-string">"World"</span>;
<span class="hljs-variable hljs-language">console</span>.<span class="hljs-title hljs-function">warn</span>(<span class="hljs-string">"Hello, "</span> + name + <span class="hljs-string">"!"</span>)</code></pre>

API

This package exports no identifiers. The default export is rehypeHighlight.

unified().use(rehypeHighlight[, options])

Apply syntax highlighting to code with highlight.js.

options

Configuration (optional).

options.prefix

Prefix to use before classes (string, default: 'hljs-').

options.detect

Whether to detect the programming language on code without a language class (boolean, default: false).

options.subset

Languages to check when automatically detecting (Array<string>, default: all languages).

options.plainText

List of plain-text languages (Array<string>, default: []). Pass any languages you would like to be kept as plain-text instead of getting highlighted. This is like setting a no-highlight class assuming txt was listed, then language-txt would be treated as such too.

options.ignoreMissing

Swallow errors for missing languages (boolean, default: false). By default, unregistered syntaxes throw an error when they are used. Pass true to swallow those errors and thus ignore code with unknown code languages.

options.aliases

Register more aliases (Record<string, string|Array<string>>, default: {}). Passed to lowlight.registerAlias.

options.languages

Register more languages (Record<string, Function>, default: {}). Each key/value pair passed as arguments to lowlight.registerLanguage.

Example

Example: ignoring

There are three ways to not apply syntax highlighting to code blocks. They can be ignored with an explicit class of no-highlight (or nohighlight), an explicit language name that’s listed in options.plainText, or by setting options.detect to false (default), which prevents <code> without a class from being automatically detected.

For example, with example.html:

<pre><code>this won’t be highlighted due to `detect: false` (default)</code></pre>

<pre><code class="no-highlight">this won’t be highlighted due to its class</code></pre>

<pre><code class="language-txt">this won’t be highlighted due to `plainText: ['txt']`</code></pre>

And example.js:

import {read} from 'to-vfile'
import {rehype} from 'rehype'
import rehypeHighlight from 'rehype-highlight'

const file = await rehype()
  .data('settings', {fragment: true})
  .use(rehypeHighlight, {plainText: ['txt', 'text']})
  .process(await read('example.html'))

console.log(String(file))

Running that yields the same as example.html: none of them are highlighted.

Example: registering

rehype-highlight supports 35 common used languages by default. This makes it small to load in browsers and Node.js, while supporting most cases by default. It’s possible to add support for more languages.

For example, with example.html:

<pre><code class="language-bnf">a ::= 'a' | 'A'</code></pre>

And example.js:

import {read} from 'to-vfile'
import {rehype} from 'rehype'
import rehypeHighlight from 'rehype-highlight'
import bnf from 'highlight.js/lib/languages/bnf'

main()

async function main() {
  const file = await rehype()
    .data('settings', {fragment: true})
    .use(rehypeHighlight, {languages: {bnf}})
    .process(await read('example.html'))

  console.log(String(file))
}

Running that yields:

<pre><code class="hljs language-bnf">a ::= <span class="hljs-string">'a'</span> | <span class="hljs-string">'A'</span></code></pre>

Example: aliases

You can map your own language flags to highlight.js languages.

For example, with example.html:

<pre><code class="language-custom-script">console.log(1)</code></pre>

And example.js:

import {read} from 'to-vfile'
import {rehype} from 'rehype'
import rehypeHighlight from 'rehype-highlight'

main()

async function main() {
  const file = await rehype()
    .data('settings', {fragment: true})
    // 👉 **Note**: the keys are registered and full highlight.js names, and
    // the values are the flags that you want to allow as `x` in `language-x`
    // classes.
    .use(rehypeHighlight, {aliases: {'javascript': 'custom-script'}})
    .process(await read('example.html'))

  console.log(String(file))
}

Running that yields:

<pre><code class="hljs language-custom-script"><span class="hljs-variable hljs-language">console</span>.<span class="hljs-title hljs-function">log</span>(<span class="hljs-number">1</span>)</code></pre>

Example: sanitation

Applying syntax highlighting in rehype operates on <code> elements with certain classes and it injects many <span> elements with classes. Allowing arbitrary classes is an opening for XSS vulnerabilities.

Working with user input and HTML generally opens you up to XSS vulnerabilities, so it’s recommend to use sanitation mechanisms, typically rehype-sanitize. Because arbitrary classes are one such opening that rehype-sanitize takes care off, using rehype-highlight with rehype-sanitize requires some configuration to make it work.

There are two ways to make it work. Either by using rehype-sanitize first while allowing the classes on <code> and then using rehype-highlight, or alternatively first using rehype-highlight and then using rehype-sanitize while allowing the classes on <span> elements. Using rehype-sanitize before rehype-highlight:

import {unified} from 'unified'
import rehypeParse from 'rehype-parse'
import rehypeHighlight from 'rehype-highlight'
import rehypeSanitize, {defaultSchema} from 'rehype-sanitize'
import rehypeStringify from 'rehype-stringify'

const file = await unified()
  .use(rehypeParse, {fragment: true})
  .use(rehypeSanitize, {
    ...defaultSchema,
    attributes: {
      ...defaultSchema.attributes,
      code: [
        ...(defaultSchema.attributes.code || []),
        // List of all allowed languages:
        ['className', 'language-js', 'language-css', 'language-md']
      ]
    }
  })
  .use(rehypeHighlight)
  .use(rehypeStringify)
  .process('<pre><code className="language-js">console.log(1)</code></pre>')

console.log(String(file))

Using rehype-highlight before rehype-sanitize:

 const file = await unified()
   .use(rehypeParse, {fragment: true})
+  .use(rehypeHighlight)
   .use(rehypeSanitize, {
     ...defaultSchema,
     attributes: {
       ...defaultSchema.attributes,
-      code: [
-        ...(defaultSchema.attributes.code || []),
-        // List of all allowed languages:
-        ['className', 'hljs', 'language-js', 'language-css', 'language-md']
+      span: [
+        ...(defaultSchema.attributes.span || []),
+        // List of all allowed tokens:
+        ['className', 'hljs-addition', 'hljs-attr', 'hljs-attribute', 'hljs-built_in', 'hljs-bullet', 'hljs-char', 'hljs-code', 'hljs-comment', 'hljs-deletion', 'hljs-doctag', 'hljs-emphasis', 'hljs-formula', 'hljs-keyword', 'hljs-link', 'hljs-literal', 'hljs-meta', 'hljs-name', 'hljs-number', 'hljs-operator', 'hljs-params', 'hljs-property', 'hljs-punctuation', 'hljs-quote', 'hljs-regexp', 'hljs-section', 'hljs-selector-attr', 'hljs-selector-class', 'hljs-selector-id', 'hljs-selector-pseudo', 'hljs-selector-tag', 'hljs-string', 'hljs-strong', 'hljs-subst', 'hljs-symbol', 'hljs-tag', 'hljs-template-tag', 'hljs-template-variable', 'hljs-title', 'hljs-type', 'hljs-variable'
+        ]
       ]
     }
   })
-  .use(rehypeHighlight)
   .use(rehypeStringify)
   .process('<pre><code className="language-js">console.log(1)</code></pre>')

Types

This package is fully typed with TypeScript. It exports an Options type, which specifies the interface of the accepted options.

Compatibility

Projects maintained by the unified collective are compatible with all maintained versions of Node.js. As of now, that is Node.js 12.20+, 14.14+, and 16.0+. Our projects sometimes work with older versions, but this is not guaranteed.

This plugin works with rehype-parse version 1+, rehype-stringify version 1+, rehype version 1+, and unified version 4+.

Security

Use of rehype-highlight should be safe to use as highlight.js and lowlight should be safe to use. When in doubt, use rehype-sanitize.

Related

Contribute

See contributing.md in rehypejs/.github for ways to get started. See support.md for ways to get help.

This project has a code of conduct. By interacting with this repository, organization, or community you agree to abide by its terms.

License

MIT © Titus Wormer