Syntax highlighting

This guide explores how to apply syntax highlighting to code blocks. MDX supports standard markdown syntax (CommonMark). It does not apply syntax highlighting to code blocks by default.

There are two ways to accomplish syntax highlighting: at compile time or at runtime. Doing it at compile time means the effort is spent upfront so that readers will have a fast experience as no extra code is sent to them (syntax highlighting needs a lot of code to work). Doing it at runtime gives more flexibility by moving the work to the client. This can result in a slow experience for readers though. It also depends on what framework you use (as in it’s specific to React, Preact, Vue, etc.)

Syntax highlighting at compile time

Use either rehype-highlight (highlight.js) or @mapbox/rehype-prism (Prism) by doing something like this:

import rehypeHighlight from 'rehype-highlight'
import {compile} from '@mdx-js/mdx'

const code = `~~~js
console.log(1)
~~~`

console.log(
  String(await compile(code, {rehypePlugins: [rehypeHighlight]}))
)

<>
  <pre>
    <code className="hljs language-js">
      <span className="hljs-built_in">console</span>.log(
      <span className="hljs-number">1</span>)
    </code>
  </pre>
</>

<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.4.0/styles/github-dark.min.css">

<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/prism/1.26.0/themes/prism-dark.min.css">

Syntax highlighting at run time

Use for example react-syntax-highlighter, by doing something like this:

import SyntaxHighlighter from 'react-syntax-highlighter'
import Post from './example.mdx' // Assumes an integration is used to compile MDX -> JS.

<Post components={{code}} />

function code({className, ...props}) {
  const match = /language-(\w+)/.exec(className || '')
  return match
    ? <SyntaxHighlighter language={match[1]} PreTag="div" {...props} />
    : <code className={className} {...props} />
}

<>
  <pre>
    <div
      className="language-js"
      style={{
        display: 'block',
        overflowX: 'auto',
        padding: '0.5em',
        background: '#F0F0F0',
        color: '#444'
      }}
    >
      <code style={{whiteSpace: 'pre'}}>
        <span>console.</span>
        <span style={{color: '#397300'}}>log</span>
        <span>(</span>
        <span style={{color: '#880000'}}>1</span>
        <span>)</span>
      </code>
    </div>
  </pre>
</>

Syntax highlighting with the meta field

Markdown supports a meta string for code:

```js filename="index.js"
console.log(1)
```

The meta part is everything after the language (in this case, js). This is a hidden part of markdown: it’s normally ignored. But as the above example shows, it’s a useful place to put some extra fields.

@mdx-js/mdx doesn’t know whether you’re handling code as a component or what the format of that meta string is, so it defaults to how markdown handles it: meta is ignored.

But what if you want to access meta? The short answer is: use remark-mdx-code-meta. It lets you type JSX attributes in the meta part which you can access by with a component for pre.

The long answer is: do it yourself, however you want, by writing a custom plugin to interpret the meta field. For example, it’s possible to pass that string as a prop with a rehype plugin:

import {visit} from 'unist-util-visit'

/** @type {import('unified').Plugin<Array<void>, import('hast').Root>} */
function rehypeMetaAsAttributes() {
  return (tree) => {
    visit(tree, 'element', (node) => {
      if (node.tagName === 'code' && node.data && node.data.meta) {
        node.properties.meta = node.data.meta
      }
    })
  }
}

This would yields the following JSX:

<>
  <pre>
    <code className="language-js" meta='filename="index.js"'>
      console.log(1)
    </code>
  </pre>
</>

The meta string in this example looks a lot like HTML attributes. What if we wanted to add each “attribute” as a prop? That can be achieved with the same rehype plugin as above with a different onelement handler:

// A regex that looks for a simplified attribute name, optionally followed
// by a double, single, or unquoted attribute value
const re = /\b([-\w]+)(?:=(?:"([^"]*)"|'([^']*)'|([^"'\s]+)))?/g

    // …
    visit(tree, 'element', (node) => {
      let match

      if (node.tagName === 'code' && node.data && node.data.meta) {
        re.lastIndex = 0 // Reset regex.

        while ((match = re.exec(node.data.meta))) {
          node.properties[match[1]] = match[2] || match[3] || match[4] || ''
        }
      }
    })
    // …

This would yields the following JSX:

<>
  <pre>
    <code className="language-js" filename="index.js">
      console.log(1)
    </code>
  </pre>
</>