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5-7 minutes read
By John Otander, Titus Wormer

What is MDX?

This article explains what the MDX format is. It shows how markdown, JSX, JavaScript expressions, and import and export statements in ESM can be used inside MDX. See § Getting started for details on how to integrate MDX into your project.

Contents

Prerequisites

To write and enjoy MDX, you should be familiar with both markdown (see this cheat sheet and tutorial for help) and JavaScript (specifically JSX).

Markdown for the component era

MDX allows you to use JSX in your markdown content. You can import components, such as interactive charts or alerts, and embed them within your content. This makes writing long-form content with components a blast. 🚀

More practically MDX can be explained as a format that combines markdown with JSX and looks as follows:

MDX
# Hello, world!

<div className="note">
  > Some notable things in a block quote!
</div>

The heading and block quote are markdown, while those HTML-like tags are JSX. Markdown often feels more natural to type than HTML or JSX for common things like emphasis or headings. JSX is an extension to JavaScript that looks like HTML but makes it convenient to use components (reusable things).

This example uses className on the <div>. That’s because it was written for React and React expects classes that way. Other frameworks, such as Vue and Preact, expect classes to be defined differently, so note that there are some differences in how JSX has to be authored depending on what tools it’s used with.

A few other features from JavaScript are supported in MDX as well: expressions in braces ({1 + 1}) and ESM (import and export).

MDX syntax

Note: You don’t have to use this syntax with @mdx-js/* packages. Or use it always. If you’re using a bundler integration you can change between MDX and markdown through the file extension (.mdx vs. .md). Alternatively, options.format can be used.

The MDX syntax combines markdown with JSX. This gives us something along the lines of literate programming. It also gives us an odd mix of two languages: markdown is whitespace sensitive and forgiving (what you type may not exactly work but it won’t crash) whereas JavaScript is whitespace insensitive and unforgiving (it does crash on typos).

Weirdly enough we quite like how they combine!

Markdown

Markdown often feels more natural to type than HTML or JSX for common things like emphasis or headings. Markdown typically looks more like what’s intended and is terser. Instead of the following HTML:

HTML
<blockquote>
  <p>A blockquote with <em>some</em> emphasis.</p>
</blockquote>

You can write the equivalent in markdown (or MDX) like so:

Markdown
> A blockquote with *some* emphasis.

MDX supports standard markdown by default (CommonMark):

Markdown
# Heading (rank 1)
## Heading 2
### 3
#### 4
##### 5
###### 6

> Block quote

* Unordered
* List

1. Ordered
2. List

A paragraph, introducing a thematic break:

---

```js
some.code()
```

a [link](https://example.com), an ![image](./image.png), some *emphasis*,
something **strong**, and finally a little `code()`.

Nonstandard markdown features (such as GFM, frontmatter, math, syntax highlighting) can be enabled with plugins (see ¶ Using plugins).

Some markdown features don’t work in MDX:

  • Indented code does not work in MDX:
    MDX
        console.log(1) // this is a paragraph in MDX!
    The reason for that is so you can nicely indent your components:
    MDX
    <main>
  <article>
    # Hello!
  </article>
</main>
  • Autolinks do not work in MDX. The reason is that they can be indistinguishable from JSX (for example: <svg:rect>) and we prefer being explicit. If you want links, use full links: [descriptive text](https://and-the-link-here.com)
  • HTML syntax doesn’t work in MDX as it’s replaced by JSX (<img> to <img />). Instead of HTML comments, you can use JavaScript comments in braces: {/* comment! */}
  • Unescaped left angle bracket / less than (<) and left curly brace ({) have to be escaped: \< or \{ (or use expressions: {'<'}, {'{'})

More on how MDX differs from markdown is documented here.

JSX

JSX is an extension to JavaScript that looks like HTML but makes it convenient to use components (reusable things). JSX is typically combined with a frontend framework like React, Preact, or Vue. These frameworks add support for components, which let you change repeating things like the following markup:

HTML
<h2>Hello, Venus!</h2>
<h2>Hello, Mars!</h2>

…to JSX (or MDX) like this:

MDX
<Welcome name="Venus" />
<Welcome name="Mars" />

JSX is good for components. It makes repeating things more clear and allows for separation of concerns. MDX supports JSX syntax. The following looks a lot like HTML:

MDX
<h1>Heading!</h1>

<abbr title="HyperText Markup Language">HTML</abbr> is a lovely language.

<section>
  And here is *markdown* in **JSX**!
</section>

But as previously mentioned you can use components too. Note that components must be defined. You can import them, define them locally, or pass them in later (see § Using MDX):

MDX
<MyComponent id="123" />

You can also use objects with components, such as the `thisOne` component on
the `myComponents` object: <myComponents.thisOne />

<Component
  open
  x={1}
  label={'this is a string, *not* markdown!'}
  icon={<Icon />}
/>

There are a few edge cases where MDX differs from JSX.

Expressions

MDX also supports JavaScript expressions inside curly braces:

MDX
Two 🍰 is: {Math.PI * 2}

Expressions can contain whole JavaScript programs as long as they’re (wrapped in) an expression that evaluates to something that can be rendered. You can use an IIFE like so:

MDX
{(function () {
  const guess = Math.random()

  if (guess > 0.66) {
    return <span style={{color: 'tomato'}}>Look at us.</span>
  }

  if (guess > 0.33) {
    return <span style={{color: 'violet'}}>Who would have guessed?!</span>
  }

  return <span style={{color: 'goldenrod'}}>Not me.</span>
})()}

Expressions can be empty or contain just a comment:

MDX
{/* A comment! */}

ESM

MDX supports import and export statements from JavaScript as well. These ESM features can be used within MDX to define things:

MDX
import {External} from './some/place.js'

export const Local = props => <span style={{color: 'red'}} {...props} />

An <External>external</External> component and a <Local>local one</Local>.

ESM can also be used for non-components (data):

MDX
import {Chart} from './chart.js'
import population from './population.js'
export const pi = 3.14

<Chart data={population} label={'Something with ' + pi} />

Interleaving

You can use markdown “inlines” but not “blocks” inside JSX if the text and tags are on the same line:

MDX
<div># this is not a heading but *this* is emphasis</div>

Text and tags on one line don’t produce blocks so they don’t produce <p>s either. On separate lines, they do:

MDX
<div>
  This is a `p`.
</div>

We differentiate using this rule (same line or not). Not based on semantics of elements in HTML. So you can build incorrect HTML (which you shouldn’t):

MDX
<h1 className="main">
  Don’t do this: it’s a `p` in an `h1`
</h1>

<h1 className="main">Do this: an `h1` with `code`</h1>

It’s not possible to wrap “blocks” if text and tags are on the same line but the corresponding tags are on different lines:

MDX
Welcome! <a href="about.html">

This is home of...

# The Falcons!</a>

That’s because to parse markdown, we first have to divide it into “blocks”. So in this case two paragraphs and a heading. Leaving an opening a tag in the first paragraph and a stray closing a tag in the heading.

Further reading