Using MDX

6-10 minutes read
By Titus Wormer

Using MDX

This article explains how to use MDX files in your project. It shows how you can pass props and how to import, define, or pass components. See § Getting started for how to integrate MDX into your project. To understand how the MDX format works, we recommend that you start with § What is MDX.

How MDX works
MDX content
MDX provider

How MDX works

An integration compiles MDX syntax to JavaScript. Say we have an MDX document, example.mdx:

input.mdx

export const Thing = () => <>World</>

# Hello <Thing />

That’s roughly turned into the following JavaScript. The below might help to form a mental model:

output-outline.jsx

/* @jsxRuntime automatic @jsxImportSource react */

export const Thing = () => <>World</>

export default function MDXContent() {
  return <h1>Hello <Thing /></h1>
}

Some observations:

The output is serialized JavaScript that still needs to be evaluated
A comment is injected to configure how JSX is handled
It’s a complete file with import/exports
A component (MDXContent) is exported

The actual output is:

output-actual.js

/* @jsxRuntime automatic @jsxImportSource react */
import {Fragment as _Fragment, jsx as _jsx, jsxs as _jsxs} from 'react/jsx-runtime'

export const Thing = () => _jsx(_Fragment, {children: 'World'})

function _createMdxContent(props) {
  const _components = {
    h1: 'h1',
    ...props.components
  }
  return _jsxs(_components.h1, {
    children: ['Hello ', _jsx(Thing, {})]
  })
}

export default function MDXContent(props = {}) {
  const {wrapper: MDXLayout} = props.components || {}
  return MDXLayout
    ? _jsx(MDXLayout, Object.assign({}, props, {children: _jsx(_createMdxContent, props)}))
    : _createMdxContent(props)
}

Some more observations:

JSX is compiled away to function calls and an import of React†
The content component can be given {components: {wrapper: MyLayout}} to wrap all content
The content component can be given {components: {h1: MyComponent}} to use something else for the heading

† MDX is not coupled to React. You can also use it with Preact, Vue, Emotion, Theme UI, etc. Both the classic and automatic JSX runtimes are supported.

MDX content

We just saw that MDX files are compiled to components. You can use those components like any other component in your framework of choice. Take this file:

example.mdx

# Hi!

It could be imported and used in a React app like so:

example.js

import React from 'react'
import ReactDom from 'react-dom'
import Example from './example.mdx' // Assumes an integration is used to compile MDX -> JS.

const root = ReactDom.createRoot(document.getElementById('root'))
root.render(<Example />)

The main content is exported as the default export. All other values are also exported. Take this example:

example.mdx

export const Thing = () => <>World</>

# Hello <Thing />

It could be imported in the following ways:

example.js

// A namespace import to get everything:
import * as everything from './example.mdx' // Assumes an integration is used to compile MDX -> JS.
console.log(everything) // {Thing: [Function: Thing], default: [Function: MDXContent]}

// Default export shortcut and a named import specifier:
import Content, {Thing} from './example.mdx'
console.log(Content) // [Function: MDXContent]
console.log(Thing) // [Function: Thing]

// Import specifier with another local name:
import {Thing as AnotherName} from './example.mdx'
console.log(AnotherName) // [Function: Thing]

Props

In § What is MDX, we showed that JavaScript expressions, inside curly braces, can be used in MDX:

example.mdx

import {year} from './data.js'
export const name = 'world'

# Hello {name.toUpperCase()}

The current year is {year}

Instead of importing or defining data within MDX, data can also be passed to MDXContent. The passed data is called props. Take for example:

example.mdx

# Hello {props.name.toUpperCase()}

The current year is {props.year}

This file could be used as:

example.jsx

import Example from './example.mdx' // Assumes an integration is used to compile MDX -> JS.

// Use a `createElement` call:
React.createElement(Example, {name: 'Venus', year: 2021})

// Use JSX:
<Example name="Mars" year={2022} />

Components

There is one special prop: components. It takes an object mapping component names to components. Take this example:

example.mdx

# Hello *<Planet />*

It can be imported from JavaScript and passed components like so:

example.jsx

import Example from './example.mdx' // Assumes an integration is used to compile MDX -> JS.

<Example components={{Planet: () => <span style={{color: 'tomato'}}>Pluto</span>}} />

You don’t have to pass components. You can also define or import them within MDX:

example.mdx

import {Box, Heading} from 'rebass'

MDX using imported components!

<Box>
  <Heading>Here’s a heading</Heading>
</Box>

Because MDX files are components, they can also import each other:

example.mdx

import License from './license.md' // Assumes an integration is used to compile MDX -> JS.
import Contributing from './docs/contributing.mdx'

# Hello world

<License />

---

<Contributing />

Here are some other examples of passing components:

example.jsx

<Example
  components={{
    // Map `h1` (`# heading`) to use `h2`s.
    h1: 'h2',
    // Rewrite `em`s (`*like so*`) to `i` with a goldenrod foreground color.
    em: (props) => <i style={{color: 'goldenrod'}} {...props} />,
    // Pass a layout (using the special `'wrapper'` key).
    wrapper: ({components, ...rest}) => <main {...rest} />,
    // Pass a component.
    Planet: () => 'Neptune',
    // This nested component can be used as `<theme.text>hi</theme.text>`
    theme: {text: (props) => <span style={{color: 'grey'}} {...props} />}
  }}
/>

The following keys can be passed in components:

HTML equivalents for the things you write with markdown such as h1 for # heading (see § Table of components for examples)
wrapper, which defines the layout (but a local layout takes precedence)
* anything else that is a valid JavaScript identifier (foo, Quote, _, $x, a1) for the things you write with JSX (like <So /> or <like.so />, note that locally defined components take precedence)‡

If you ever wondered what the rules are for whether a name in JSX (so x in <x>) is a literal tag name (like h1) or not (like Component), they are as follows:

If there’s a dot, it’s a member expression (<a.b> -> h(a.b)), which means that the b component is taken from object a
Otherwise, if the name is not a valid identifier, it’s a literal (<a-b> -> h('a-b'))
Otherwise, if it starts with a lowercase, it’s a literal (<a> -> h('a'))
Otherwise, it’s an identifier (<A> -> h(A)), which means a component A

Layout

There is one special component: the layout. If it is defined, it’s used to wrap all content. A layout can be defined from within MDX using a default export:

MDX

export default function Layout({children}) {
  return <main>{children}</main>;
}

All the things.

The layout can also be imported and then exported with an export … from:

MDX

export {Layout as default} from './components.js'

The layout can also be passed as components.wrapper (but a local one takes precedence).

MDX provider

You probably don’t need a provider. Passing components is typically fine. Providers often only add extra weight. Take for example this file:

post.mdx

# Hello world

Used like so:

app.jsx

import React from 'react'
import ReactDom from 'react-dom'
import Post from './post.mdx' // Assumes an integration is used to compile MDX -> JS.
import {Heading, /* … */ Table} from './components/index.js'

const components = {
  h1: Heading.H1,
  // …
  table: Table
}

const root = ReactDom.createRoot(document.getElementById('root'))
root.render(<Post components={components} />)

That works, those components are used.

But when you’re nesting MDX files (importing them into each other) it can become cumbersome. Like so:

post.mdx

import License from './license.md' // Assumes an integration is used to compile MDX -> JS.
import Contributing from './docs/contributing.mdx'

# Hello world

<License components={props.components} />

---

<Contributing components={props.components} />

To solve this, a context can be used in React, Preact, and Vue. Context provides a way to pass data through the component tree without having to pass props down manually at every level. Set it up like so:

Install either @mdx-js/react, @mdx-js/preact, or @mdx-js/vue, depending on what framework you’re using
Configure your MDX integration with options.providerImportSource set to that package, so either '@mdx-js/react', '@mdx-js/preact', or '@mdx-js/vue'
Import MDXProvider from that package. Use it to wrap your top-most MDX content component and pass it your components instead:

Diff

 import React from 'react'
 import ReactDom from 'react-dom'
 import Post from './post.mdx' // Assumes an integration is used to compile MDX -> JS.
 import {Heading, /* … */ Table} from './components/index.js'
+import {MDXProvider} from '@mdx-js/react'

 const components = {
   h1: Heading.H1,
   // …
   table: Table
 }

 const root = ReactDom.createRoot(document.getElementById('root'))
-root.render(<Post components={components} />)
+root.render(
+  <MDXProvider components={components}>
+    <Post />
+  </MDXProvider>
+)

Now you can remove the explicit and verbose component passing:

Diff

 import License from './license.md' // Assumes an integration is used to compile MDX -> JS.
 import Contributing from './docs/contributing.mdx'

 # Hello world

-<License components={props.components} />
+<License />

 ---

-<Contributing components={props.components} />
+<Contributing />

When MDXProviders are nested, their components are merged. Take this example:

TypeScript

<>
  <MDXProvider components={{h1: Component1, h2: Component2}}>
    <MDXProvider components={{h2: Component3, h3: Component4}}>
      <Content />
    </MDXProvider>
  </MDXProvider>
</>

…which results in h1s using Component1, h2s using Component3, and h3s using Component4.

To merge differently or not at all, pass a function to components. It’s given the current context components and what it returns will be used instead. In this example the current context components are discarded:

TypeScript

<>
  <MDXProvider components={{h1: Component1, h2: Component2}}>
    <MDXProvider components={() => ({h2: Component3, h3: Component4})}>
      <Content />
    </MDXProvider>
  </MDXProvider>
</>

…which results in h2s using Component3 and h3s using Component4. No component is used for h1.

If you’re not nesting MDX files, or not nesting them often, don’t use providers: pass components explicitly.

Using MDX

Contents

How MDX works

MDX content

Props

Components

Layout

MDX provider