This post is quite old and no longer being updated. The following information may no longer work the with the latest version of the tools, libraries, frameworks, or best-practices discussed. Here is some fancy syntax highlighting using `react-syntax-highlighter` and `react-markdown` in Next.js! ```tsx{5} const Hello = () => { return (

Let's dive into syntax highlighting!

) } export default Hello; ``` So how exactly is it done? Most info about this references a [now deprecated](https://github.com/remarkjs/react-markdown/blob/main/changelog.md#600---2021-04-15) way of doing this (`renderer` is no longer a thing). With a bit of digging through [the docs](https://github.com/remarkjs/react-markdown#use-custom-components-syntax-highlight) here's how you can get code syntax highlighting in Next.js (and Typescript). ### Install Dependencies `npm i react-markdown react-syntax-highlighter` If you're using TypeScript, you will also want the Types. The following command will add the types package as a development dependency: `npm i @types/react-syntax-highlighter -D` ### Configure the `ReactMarkdown` Component I will assume you already have some page or component where you are processing Markdown. In the case of `react-markdown` you may have a component called `Markdown.tsx` that looks something like this: ```tsx // Markdown.tsx import { FC } from 'react; import ReactMarkdown from 'react-markdown'; type MarkdownProps = { markdown: string & { content?: string }; }; const Markdown: FC = ({ markdown }) => { return ( {markdown.content} ) } export default Markdown; ``` In this basic example, the `ReactMarkdown` component is being passed raw markdown with the prop `markdown.content`. You can change this to work for your project. From here, let's simply add the `components` prop to `ReactMarkdown`. This gives us the ability to add `SyntaxHighlighter` in the next steps. ```tsx{12-14,18} // Markdown.tsx import { FC } from 'react; import ReactMarkdown from 'react-markdown'; type MarkdownProps = { markdown: string & { content?: string }; }; const Markdown: FC = ({ markdown }) => { const MarkdownComponents: object = { // SyntaxHighlight code will go here } return ( {markdown.content} ) } export default Markdown; ``` ### Choose the Right SyntaxHighlighter Import for Your Project The [official documentation](https://github.com/react-syntax-highlighter/react-syntax-highlighter#react-syntax-highlighter) covers this quite well, but here's the tl;dr: - Unless you want a massive JS bundle on your markdown pages, you should only be using the [Light Build](https://github.com/react-syntax-highlighter/react-syntax-highlighter#light-build). You will need to manually import the specific languages you are going to be syntax highlighting in your markdown. - If you're using Next.js, change the theme import from `…/dist/esm/…` to `…/dist/cjs/…`. Hopefully ESM is fully [supported in Next.js](https://github.com/vercel/next.js/issues/9607) soon. - If you want to syntax highlight TSX/JSX you must use Prism. Highlight.js does not support TSX or JSX. For the rest of this guide, I will be using the Prism Light Build with cjs imports. I will add TSX, TypeScript, SCSS, Bash, Markdown, and JSON support. Simply add whichever languages you'd like to use. [Check here](https://github.com/react-syntax-highlighter/react-syntax-highlighter/blob/master/AVAILABLE_LANGUAGES_PRISM.MD) for languages supported by Prism. ```tsx{4-18} // Markdown.tsx import { FC } from 'react; import ReactMarkdown from 'react-markdown'; import { PrismLight as SyntaxHighlighter } from 'react-syntax-highlighter'; import tsx from 'react-syntax-highlighter/dist/cjs/languages/prism/tsx'; import typescript from 'react-syntax-highlighter/dist/cjs/languages/prism/typescript'; import scss from 'react-syntax-highlighter/dist/cjs/languages/prism/scss'; import bash from 'react-syntax-highlighter/dist/cjs/languages/prism/bash'; import markdown from 'react-syntax-highlighter/dist/cjs/languages/prism/markdown'; import json from 'react-syntax-highlighter/dist/cjs/languages/prism/json'; SyntaxHighlighter.registerLanguage('tsx', tsx); SyntaxHighlighter.registerLanguage('typescript', typescript); SyntaxHighlighter.registerLanguage('scss', scss); SyntaxHighlighter.registerLanguage('bash', bash); SyntaxHighlighter.registerLanguage('markdown', markdown); SyntaxHighlighter.registerLanguage('json', json); const Markdown: FC = ({ markdown }) => { type MarkdownProps = { markdown: string & { content?: string }; }; const MarkdownComponents: object = { // SyntaxHighlight code will go here } return ( {markdown.content} ) } export default Markdown; ``` ### Configure the `SyntaxHighlighter` component and add Line Highlighting Functionality Let's finally start adding code for our `ReactMarkdown` custom component. We will import a [theme](https://github.com/react-syntax-highlighter/react-syntax-highlighter/blob/master/AVAILABLE_STYLES_PRISM.MD) and `parse-numeric-range` for the line highlight logic. ```tsx{4-5,13,14-55} // Markdown.tsx ... import rangeParser from 'parse-numeric-range'; import { oneDark } from 'react-syntax-highlighter/dist/cjs/styles/prism'; type MarkdownProps = { markdown: string & { content?: string }; }; const Markdown: FC = ({ markdown }) => { const syntaxTheme = oneDark; const MarkdownComponents: object = { code({ node, inline, className, ...props }) { const hasLang = /language-(\w+)/.exec(className || ''); const hasMeta = node?.data?.meta; const applyHighlights: object = (applyHighlights: number) => { if (hasMeta) { const RE = /{([\d,-]+)}/; const metadata = node.data.meta?.replace(/\s/g, ''); const strlineNumbers = RE?.test(metadata) ? RE?.exec(metadata)[1] : '0'; const highlightLines = rangeParser(strlineNumbers); const highlight = highlightLines; const data: string = highlight.includes(applyHighlights) ? 'highlight' : null; return { data }; } else { return {}; } }; return hasLang ? ( {props.children} ) : (


      )
    },
  }

  ...
}

export default Markdown;
```

- First we check for the existence of the metadata to prevent TypeScript errors if it's undefined.
- Inside of the `applyHighlights` function, we declare `RE` as our Regex checker.
- Then we declare a variable called `metadata` that removes any spaces from the object.
- `strlineNumbers` checks the metadata against the Regex and ensures the object contains only numbers, hyphens, commas. 
- `rangeParser()` spits out an array of line numbers based upon the range we defined in markdown!
- We then check which lines to highlight and apply a data attribute `data="highlight"` on the spans wrap the lines we defined.

We now have the ability to style `data="highlight"` with some CSS. I will leave this up to you, as it will be highly dependent on the look and style you are going for.

### Controlling Line Highlighting in Your Markdown

So now that you have your component code in place for `React-Markdown` and `SyntaxHighlighter`, how do you actually highlight lines of code within your markdown?

```markdown
```tsx {3-4, 8}
```

Simply add curly braces with comma separated line numbers and/or ranges of line numbers!

### Adding Custom CSS

You can easily target your `ReactMarkdown` and `SyntaxHighlighter` components like so. You may have to use some `!important` tags to target certain elements within the Syntax Highlighting, which is less than ideal, but it works.


	The following example is CSS-in-JS (Emotion specifically). Notice the class names and refactor the following for your CSS setup. I plan to update this guide to use Tailwind CSS in the future.


```tsx{10-29,37,47}
// Markdown.tsx
...

const Markdown: FC = ({ markdown }) => {

  ...

  const MarkdownComponents: object = {

    const styleMarkdown = css({
      '.codeStyle, pre, code, code span': {
        // Your SyntaxHighlighter override styles here
      },
      code: {
       // Your general code styles here
      },
      'pre code': {
        // Your code-block styles here
      },
      'h3 code': {
        color: 'inherit'
      },
      'span.linenumber': {
        display: 'none !important'
      },
      '[data="highlight"]': {
        // Your custom line highlight styles here
      },
    })

    code({ node, inline, className, ...props }) {
     
      ...

      return hasLang ? (
        
          {props.children}
        
      )
    }
  }

  return (
    
      {markdown.content}
    
  )
}

...
```



### Fixing Mobile Issues

There is a known issue with Prism where your CSS background color for the line highlight will not extend past the viewport. So when you scroll horizontally on a mobile screen, the highlighting abruptly ends.

There are a lot of over-engineered recommendations to fix it, including wrapping the pre tag in a container to apply more styles. The fix I came up with is hacky, yet elegant:

```scss
code: {
  transform: translateZ(0);
  min-width: 100%;
  float: left;
  & > span {
    display: block;
  }
}
```

I had almost forgotten about floats but hey... it works 🤷


### Removing Line Numbers

The lineNumber prop _must_ be set to `true` or the line highlighting won't work at all. If you'd like to disable the line numbers, this should do the trick:

```scss
span.linenumber { display: none; }
```

A better solution would be to write some additional functions to accept more meta strings for doing things like setting titles, disabling/enabling line numbers, etc. but I'll leave that for another day (and a future guide).

Whew, there you have it. Syntax highlighting your markdown in React using TypeScript with custom line highlighting. 🙌

---

Thanks to [Prince](https://prince.dev/highlight-with-react) for the excellent writeup and function from which I took inspiration. His guide is fantastic if you are using Gatsby and/or `prism-react-renderer,`

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