Frontmatter

2 minutes read
By Titus Wormer

This guide explores how to support YAML frontmatter in MDX. MDX supports standard markdown syntax (CommonMark). That means frontmatter is not supported by default.

MDX comes with a powerful and dynamic alternative to frontmatter, namely ESM (import/export). These exports:

example.mdx

export const name = 'World'
export const title = 'Hi, ' + name + '!'

# {title}

Can be used like so:

example.js

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

console.log(Post.title) // Prints 'Hi, World!'

(alias) module "*.mdx"
import Post

An MDX file which exports a JSX component.

The default export of MDX files is a function which takes props and returns a JSX element. MDX files can export other identifiers from within the MDX file as well, either authored manually or automatically through plugins

It’s currently not possible for the other exports to be typed automatically. You can type them yourself with a TypeScript script which augments *.mdx modules. A script file is a file which doesn’t use top-level ESM syntax, but ESM syntax is allowed inside the declared module.

This is typically useful for exports created by plugins. For example:

// mdx-custom.d.ts
declare module '*.mdx' {
  import { Frontmatter } from 'my-frontmatter-types';

  export const frontmatter: Frontmatter;
  export const title: string;
}

The previous example added types to all .mdx files. To define types for a specific MDX file, create a file with the same name but postfixed with .d.ts next to the MDX file.

For example, given the following MDX file my-component.mdx:

export const message = 'world';

# Hello {message}

Create the following file named my-component.mdx.d.ts in the same directory:

export { default } from '*.mdx';

export const message: string;

Note that this overwrites the declare module '*.mdx' { … } types from earlier, which is why you also need to define the default export. You can also define your own default export type to narrow the accepted prop types of this specific file.

It should now be possible to import both the MDX component and the exported constant message.

namespace console
var console: Console

The console module provides a simple debugging console that is similar to the JavaScript console mechanism provided by web browsers.

The module exports two specific components:

A Console class with methods such as console.log(), console.error() and console.warn() that can be used to write to any Node.js stream.
A global console instance configured to write to process.stdout and process.stderr. The global console can be used without calling require('console').

Warning: The global console object's methods are neither consistently synchronous like the browser APIs they resemble, nor are they consistently asynchronous like all other Node.js streams. See the note on process I/O for more information.

Example using the global console:

console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
//   Error: Whoops, something bad happened
//     at [eval]:5:15
//     at Script.runInThisContext (node:vm:132:18)
//     at Object.runInThisContext (node:vm:309:38)
//     at node:internal/process/execution:77:19
//     at [eval]-wrapper:6:22
//     at evalScript (node:internal/process/execution:76:60)
//     at node:internal/main/eval_string:23:3

const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr

Example using the Console class:

const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);

myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err

const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err

@see source

(method) Console.log(message?: any, ...optionalParams: any[]): void

Prints to stdout with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to printf(3) (the arguments are all passed to util.format()).

const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout

See util.format() for more information.

@since v0.1.100

(alias) module "*.mdx"
import Post

An MDX file which exports a JSX component.

This is typically useful for exports created by plugins. For example:

// mdx-custom.d.ts
declare module '*.mdx' {
  import { Frontmatter } from 'my-frontmatter-types';

  export const frontmatter: Frontmatter;
  export const title: string;
}

The previous example added types to all .mdx files. To define types for a specific MDX file, create a file with the same name but postfixed with .d.ts next to the MDX file.

For example, given the following MDX file my-component.mdx:

export const message = 'world';

# Hello {message}

Create the following file named my-component.mdx.d.ts in the same directory:

export { default } from '*.mdx';

export const message: string;

It should now be possible to import both the MDX component and the exported constant message.

const title: string

You might prefer frontmatter though, as it lets you define data that can be extracted from the file system before compiling. Say our MDX with frontmatter looked like this:

example.mdx

---
title: Hi, World!
---

# Hi, World!

Then without compiling or evaluating the metadata can be accessed like so:

example.js

import {read} from 'to-vfile'
import {matter} from 'vfile-matter'

const file = await read('example.mdx')
matter(file)

console.log(file.data.matter)

(alias) function read(description: Compatible, options: BufferEncoding | ReadOptions | null | undefined, callback: Callback): undefined (+2 overloads)
import read

(alias) function matter(file: VFile, options?: Options | null | undefined): undefined
import matter

const file: VFile

(alias) read(description: Compatible, options?: BufferEncoding | ReadOptions | null | undefined): Promise<VFile> (+2 overloads)
import read

(alias) matter(file: VFile, options?: Options | null | undefined): undefined
import matter

const file: VFile

namespace console
var console: Console

The console module provides a simple debugging console that is similar to the JavaScript console mechanism provided by web browsers.

The module exports two specific components:

A Console class with methods such as console.log(), console.error() and console.warn() that can be used to write to any Node.js stream.
A global console instance configured to write to process.stdout and process.stderr. The global console can be used without calling require('console').

Example using the global console:

console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
//   Error: Whoops, something bad happened
//     at [eval]:5:15
//     at Script.runInThisContext (node:vm:132:18)
//     at Object.runInThisContext (node:vm:309:38)
//     at node:internal/process/execution:77:19
//     at [eval]-wrapper:6:22
//     at evalScript (node:internal/process/execution:76:60)
//     at node:internal/main/eval_string:23:3

const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr

Example using the Console class:

const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);

myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err

const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err

@see source

(method) Console.log(message?: any, ...optionalParams: any[]): void

const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout

See util.format() for more information.

@since v0.1.100

const file: VFile

(property) VFile.data: Data

unknown

Our compiler, @mdx-js/mdx, doesn’t understand YAML frontmatter by default but it can be enabled by using a remark plugin, remark-frontmatter:

example.js

import fs from 'node:fs/promises'
import {compile} from '@mdx-js/mdx'
import remarkFrontmatter from 'remark-frontmatter'

const file = await compile(await fs.readFile('example.mdx'), {
  remarkPlugins: [remarkFrontmatter]
})

console.log(file)

(alias) module "node:fs/promises"
import fs

(alias) function compile(vfileCompatible: Readonly<Compatible>, compileOptions?: Readonly<CompileOptions> | null | undefined): Promise<VFile>
import compile

(alias) function remarkFrontmatter(options?: Options | null | undefined): undefined
import remarkFrontmatter

const file: VFile

(alias) compile(vfileCompatible: Readonly<Compatible>, compileOptions?: Readonly<CompileOptions> | null | undefined): Promise<VFile>
import compile

(alias) module "node:fs/promises"
import fs

function readFile(path: PathLike | fs.FileHandle, options?: ({
    encoding?: null | undefined;
    flag?: OpenMode | undefined;
} & EventEmitter<T extends EventMap<T> = DefaultEventMap>.Abortable) | null): Promise<Buffer> (+2 overloads)

Asynchronously reads the entire contents of a file.

If no encoding is specified (using options.encoding), the data is returned as a Buffer object. Otherwise, the data will be a string.

If options is a string, then it specifies the encoding.

When the path is a directory, the behavior of fsPromises.readFile() is platform-specific. On macOS, Linux, and Windows, the promise will be rejected with an error. On FreeBSD, a representation of the directory's contents will be returned.

An example of reading a package.json file located in the same directory of the running code:

import { readFile } from 'node:fs/promises';
try {
  const filePath = new URL('./package.json', import.meta.url);
  const contents = await readFile(filePath, { encoding: 'utf8' });
  console.log(contents);
} catch (err) {
  console.error(err.message);
}

It is possible to abort an ongoing readFile using an AbortSignal. If a request is aborted the promise returned is rejected with an AbortError:

import { readFile } from 'node:fs/promises';

try {
  const controller = new AbortController();
  const { signal } = controller;
  const promise = readFile(fileName, { signal });

  // Abort the request before the promise settles.
  controller.abort();

  await promise;
} catch (err) {
  // When a request is aborted - err is an AbortError
  console.error(err);
}

Aborting an ongoing request does not abort individual operating system requests but rather the internal buffering fs.readFile performs.

Any specified FileHandle has to support reading.

@since v10.0.0
@param path filename or FileHandle
@return Fulfills with the contents of the file.

(property) remarkPlugins?: PluggableList | null | undefined

(alias) function remarkFrontmatter(options?: Options | null | undefined): undefined
import remarkFrontmatter

namespace console
var console: Console

The console module provides a simple debugging console that is similar to the JavaScript console mechanism provided by web browsers.

The module exports two specific components:

A Console class with methods such as console.log(), console.error() and console.warn() that can be used to write to any Node.js stream.
A global console instance configured to write to process.stdout and process.stderr. The global console can be used without calling require('console').

Example using the global console:

console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
//   Error: Whoops, something bad happened
//     at [eval]:5:15
//     at Script.runInThisContext (node:vm:132:18)
//     at Object.runInThisContext (node:vm:309:38)
//     at node:internal/process/execution:77:19
//     at [eval]-wrapper:6:22
//     at evalScript (node:internal/process/execution:76:60)
//     at node:internal/main/eval_string:23:3

const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr

Example using the Console class:

const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);

myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err

const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err

@see source

(method) Console.log(message?: any, ...optionalParams: any[]): void

const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout

See util.format() for more information.

@since v0.1.100

const file: VFile

Now it “works”. The frontmatter is not rendered as if it was markdown. But the data embedded in the frontmatter isn’t available from inside the MDX. What if we wanted that too? Like so:

example.mdx

---
title: Hi, World!
---

# {title}

That’s exactly what the remark plugin remark-mdx-frontmatter does.

That plugin, like all remark plugins, can be passed as remarkPlugins in ProcessorOptions. More info on plugins is available in § Extending MDX

Frontmatter

Notes

Notes