Adding Filenames To MDX Code Blocks with Next.js MDX Components

Sometimes I finish writing some code and it just brings me happiness. That is what happened over the past few days while I was writing my previous blog post. For the longest time I've wanted to add a header to code blocks so that I could attach a filename to help with understanding where the code is meant to written to. I had tried to use some different Rehype and Remark plugins in the past but they never quite worked as I wanted. This time around I told myself I was going to figure out how to make it happen and I'm super happy with the results. Here is the finished result.

```md:blog-post.mdx
This is a code block with a filename!
```

```md:blog-post.mdx
This is a code block with a filename!
```

rehype-code-titles#

I ended up using rehype-code-titles to add this functionality. It does exactly what I needed it to do but a caveat is that it adds a completely unstyled div element. In the end, that is good because we don't have to fight to override styles, but it look a little bit to figure out how to get the styling correct.

Adding rehype-code-titles to Next.js is very easy using the createMDX functionality from @next/mdx.

import createMDX from "@next/mdx";
import rehypeCodeTitles from "rehype-code-titles";

const withMDX = createMDX({
  options: {
    rehypePlugins: [rehypeCodeTitles],
  },
});

import createMDX from "@next/mdx";
import rehypeCodeTitles from "rehype-code-titles";

const withMDX = createMDX({
  options: {
    rehypePlugins: [rehypeCodeTitles],
  },
});

The Output documentation from rehype-code-titles shows that the HTML that is generated looks like this.

<div class="rehype-code-title">lib/mdx.ts</div>
<pre>
  <code class="language-typescript">
  <!-- HTML parse code here -->
  </code>
</pre>

<div class="rehype-code-title">lib/mdx.ts</div>
<pre>
  <code class="language-typescript">
  <!-- HTML parse code here -->
  </code>
</pre>

This is great, we have a div that has our filename in it! However the HTML output by my blog is a little more complex due to having light and dark themes provided by the bright package. The HTML that is output for this site looks like this.

<div class="rehype-code-title">lib/mdx.ts</div>
<div data-bright-theme="solarized-dark" data-bright-mode="dark" style="...">
  <pre> ... </pre>
</div>
<div data-bright-theme="solarized-light" data-bright-mode="light" style="...">
  <pre> ... </pre>
</div>

<div class="rehype-code-title">lib/mdx.ts</div>
<div data-bright-theme="solarized-dark" data-bright-mode="dark" style="...">
  <pre> ... </pre>
</div>
<div data-bright-theme="solarized-light" data-bright-mode="light" style="...">
  <pre> ... </pre>
</div>

The <div data-bright-theme="..." data-bright-mode="..." style="..."> elements add margin to separate themselves from sibling elements so the resulting div from rehype-code-titles looked quite out of place. I ended up using v0 and pasting in a screenshot of what I was trying to accomplish. I'll admit that my knowledge of correct terminology for CSS questions is lacking and I learned a lot about how to target specific elements through this conversation. I think that AI having the ability to interpret UI screenshots and lead a conversation to the correct terminology is such a great experience!

MDX Components#

I haven't talked much about how the MDX Components feature of Next.js has played an important role in all of this. As I was iterating on the styling for the <div class="rehype-code-title"> I wanted to be able to add a background color to the entire div and then have a different color encompass just the text portion that was being shown. I was finding this incredibly difficult to solve and multiple attempts of different ::before and ::after pseudo elements ended up failing (could have been skill issues). That is when I realized that I could create and render my own component.

I went ahead and made a change that whenever a div is rendered, check to see if it has the rehype-code-title class. If so, we'll render a custom component otherwise we'll render the div as it was.

export const mdxComponents: MDXComponents = {
  // ...
  div: ({ className, children, ...props }) => {
		if (className?.includes("rehype-code-title")) {
			return <CodeblockTitle {...props}>{children}</CodeblockTitle>;
		}
		return (
			<div className={className} {...props}>
				{children}
			</div>
		);
	},
  // ...
}

export const mdxComponents: MDXComponents = {
  // ...
  div: ({ className, children, ...props }) => {
		if (className?.includes("rehype-code-title")) {
			return <CodeblockTitle {...props}>{children}</CodeblockTitle>;
		}
		return (
			<div className={className} {...props}>
				{children}
			</div>
		);
	},
  // ...
}

I went ahead and created the CodeblockTitle component where I inserted a span element inside of the div to give myself the ability to add a different colored background to the text portion.

export function CodeblockTitle({
	className,
	children,
	...props
}: React.HTMLAttributes<HTMLDivElement>) {
	return (
		<div
			className="rehype-code-title pt-4 px-4 pb-0 rounded-t-md mb-0
			text-sm font-normal bg-solaralizedlight-darkened dark:bg-solaralizeddark-darkened"
			{...props}
		>
			<span
			className="rehype-code-title-content relative p-1 -left-[6px] top-[6px]
			inline-block whitespace-nowrap overflow-x-auto max-w-full
			bg-solaralizedlight dark:bg-solaralizeddark"
			>
				{children}
			</span>
		</div>
	);
}

export function CodeblockTitle({
	className,
	children,
	...props
}: React.HTMLAttributes<HTMLDivElement>) {
	return (
		<div
			className="rehype-code-title pt-4 px-4 pb-0 rounded-t-md mb-0
			text-sm font-normal bg-solaralizedlight-darkened dark:bg-solaralizeddark-darkened"
			{...props}
		>
			<span
			className="rehype-code-title-content relative p-1 -left-[6px] top-[6px]
			inline-block whitespace-nowrap overflow-x-auto max-w-full
			bg-solaralizedlight dark:bg-solaralizeddark"
			>
				{children}
			</span>
		</div>
	);
}

Conclusion#

I had a ton of fun implementing this, learned a bit about CSS, and this blog looks, in my opinion, a whole lot cooler being able to have the filename next to the code block. Sometimes it's the little things that bring us the most happiness!