mdast-util-frontmatter

mdast extensions to parse and serialize frontmatter (YAML, TOML, and more).

Contents

• What is this?
• When to use this
• Install
• Use
• API
  • frontmatterFromMarkdown(options?)
  • frontmatterToMarkdown(options?)
• Syntax tree
  • Nodes
  • Content model
• Types
• Compatibility
• Related
• Contribute
• License

What is this?

This package contains extensions that add support for the frontmatter syntax enabled by GitHub and many other places to mdast-util-from-markdown and mdast-util-to-markdown.

Frontmatter is a metadata format in front of content. It’s typically written in YAML and is often used with markdown. Frontmatter does not work everywhere so it makes markdown less portable.

When to use this

These tools are all rather low-level. In most cases, you’d want to use remark-frontmatter with remark instead.

When working with mdast-util-from-markdown, you must combine this package with micromark-extension-frontmatter.

Install

This package is ESM only. In Node.js (version 12.20+, 14.14+, or 16.0+), install with npm:

npm install mdast-util-frontmatter

In Deno with esm.sh:

import {frontmatterFromMarkdown, frontmatterToMarkdown} from 'https://esm.sh/mdast-util-frontmatter@1'

In browsers with esm.sh:

<script type="module">
  import {frontmatterFromMarkdown, frontmatterToMarkdown} from 'https://esm.sh/mdast-util-frontmatter@1?bundle'
</script>

Use

Say our document example.md contains:

+++
title = "New Website"
+++

# Other markdown

…and our module example.js looks as follows:

import fs from 'node:fs/promises'
import {fromMarkdown} from 'mdast-util-from-markdown'
import {toMarkdown} from 'mdast-util-to-markdown'
import {frontmatter} from 'micromark-extension-frontmatter'
import {frontmatterFromMarkdown, frontmatterToMarkdown} from 'mdast-util-frontmatter'

const doc = await fs.readFile('example.md')

const tree = fromMarkdown(doc, {
  extensions: [frontmatter(['yaml', 'toml'])],
  mdastExtensions: [frontmatterFromMarkdown(['yaml', 'toml'])]
})

console.log(tree)

const out = toMarkdown(tree, {extensions: [frontmatterToMarkdown(['yaml', 'toml'])]})

console.log(out)

…now running node example.js yields (positional info removed for brevity):

{
  type: 'root',
  children: [
    {type: 'toml', value: 'title = "New Website"'},
    {
      type: 'heading',
      depth: 1,
      children: [{type: 'text', value: 'Other markdown'}]
    }
  ]
}

+++
title = "New Website"
+++

# Other markdown

API

This package exports the identifiers frontmatterFromMarkdown and frontmatterToMarkdown. There is no default export.

frontmatterFromMarkdown(options?)

Function that can be called to get an extension for mdast-util-from-markdown.

options

Configuration (optional). Same as micromark-extension-frontmatter.

frontmatterToMarkdown(options?)

Function that can be called to get an extension for mdast-util-to-markdown.

options

Configuration (optional). Same as micromark-extension-frontmatter.

Syntax tree

The following interfaces are added to mdast by this utility.

Nodes

👉 Note: other nodes are not enabled by default, but when passing options to enable them, they work the same as YAML.

YAML

interface YAML <: Literal {
  type: "yaml"
}

YAML (Literal) represents a collection of metadata for the document in the YAML data serialisation language.

YAML can be used where frontmatter content is expected. Its content is represented by its value field.

For example, the following markdown:

---
foo: bar
---

Yields:

{type: 'yaml', value: 'foo: bar'}

Content model

FrontmatterContent

type FrontmatterContent = YAML

Frontmatter content represent out-of-band information about the document.

If frontmatter is present, it must be limited to one node in the tree, and can only exist as a head.

FlowContent (frontmatter)

type FlowContentFrontmatter = FrontmatterContent | FlowContent

Types

This package is fully typed with TypeScript. It exports the additional types Options, Matter, and Info.

The YAML node type is supported in @types/mdast by default. To add other node types, register them by adding them to FrontmatterContentMap:

import type {Literal} from 'mdast'

interface TOML extends Literal {
  type: 'toml'
}

declare module 'mdast' {
  interface FrontmatterContentMap {
    // Allow using TOML nodes defined by `mdast-util-frontmatter`.
    toml: TOML
  }
}

Compatibility

Projects maintained by the unified collective are compatible with all maintained versions of Node.js. As of now, that is Node.js 12.20+, 14.14+, and 16.0+. Our projects sometimes work with older versions, but this is not guaranteed.

This plugin works with mdast-util-from-markdown version 1+ and mdast-util-to-markdown version 1+.

Related

• remarkjs/remark-frontmatter — remark plugin to support frontmatter
• micromark/micromark-extension-frontmatter — micromark extension to parse frontmatter

Contribute

See contributing.md in syntax-tree/.github for ways to get started. See support.md for ways to get help.

This project has a code of conduct. By interacting with this repository, organization, or community you agree to abide by its terms.

License

MIT © Titus Wormer