mdast-util-to-hast

mdast utility to transform to hast.

Contents

• What is this?
• When should I use this?
• Install
• Use
• API
  • defaultFootnoteBackContent(referenceIndex, rereferenceIndex)
  • defaultFootnoteBackLabel(referenceIndex, rereferenceIndex)
  • defaultHandlers
  • toHast(tree[, options])
  • FootnoteBackContentTemplate
  • FootnoteBackLabelTemplate
  • Handler
  • Handlers
  • Options
  • Raw
  • State
• Examples
  • Example: supporting HTML in markdown naïvely
  • Example: supporting HTML in markdown properly
  • Example: footnotes in languages other than English
  • Example: supporting custom nodes
• Algorithm
  • Default handling
  • Fields on nodes
• CSS
• Syntax tree
  • Nodes
• Types
• Compatibility
• Security
• Related
• Contribute
• License

What is this?

This package is a utility that takes an mdast (markdown) syntax tree as input and turns it into a hast (HTML) syntax tree.

When should I use this?

This project is useful when you want to deal with ASTs and turn markdown to HTML.

The hast utility hast-util-to-mdast does the inverse of this utility. It turns HTML into markdown.

The remark plugin remark-rehype wraps this utility to also turn markdown to HTML at a higher-level (easier) abstraction.

Install

This package is ESM only. In Node.js (version 16+), install with npm:

npm install mdast-util-to-hast

In Deno with esm.sh:

import {toHast} from 'https://esm.sh/mdast-util-to-hast@13'

In browsers with esm.sh:

<script type="module">
  import {toHast} from 'https://esm.sh/mdast-util-to-hast@13?bundle'
</script>

Use

Say we have the following example.md:

## Hello **World**!

…and next to it a module example.js:

import {fs} from 'node:fs/promises'
import {toHtml} from 'hast-util-to-html'
import {fromMarkdown} from 'mdast-util-from-markdown'
import {toHast} from 'mdast-util-to-hast'

const markdown = String(await fs.readFile('example.md'))
const mdast = fromMarkdown(markdown)
const hast = toHast(mdast)
const html = toHtml(hast)

console.log(html)

…now running node example.js yields:

<h2>Hello <strong>World</strong>!</h2>

API

This package exports the identifiers defaultFootnoteBackContent, defaultFootnoteBackLabel, defaultHandlers, and toHast. There is no default export.

defaultFootnoteBackContent(referenceIndex, rereferenceIndex)

Generate the default content that GitHub uses on backreferences.

Parameters

• referenceIndex (number) — index of the definition in the order that they are first referenced, 0-indexed
• rereferenceIndex (number) — index of calls to the same definition, 0-indexed

Returns

Content (Array<ElementContent>).

defaultFootnoteBackLabel(referenceIndex, rereferenceIndex)

Generate the default label that GitHub uses on backreferences.

Parameters

• referenceIndex (number) — index of the definition in the order that they are first referenced, 0-indexed
• rereferenceIndex (number) — index of calls to the same definition, 0-indexed

Returns

Label (string).

defaultHandlers

Default handlers for nodes (Handlers).

toHast(tree[, options])

Transform mdast to hast.

Parameters

• tree (MdastNode) — mdast tree
• options (Options, optional) — configuration

Returns

hast tree (HastNode).

Notes

HTML

Raw HTML is available in mdast as html nodes and can be embedded in hast as semistandard raw nodes. Most utilities ignore raw nodes but two notable ones don’t:

• hast-util-to-html also has an option allowDangerousHtml which will output the raw HTML. This is typically discouraged as noted by the option name but is useful if you completely trust authors
• hast-util-raw can handle the raw embedded HTML strings by parsing them into standard hast nodes (element, text, etc). This is a heavy task as it needs a full HTML parser, but it is the only way to support untrusted content

Footnotes

Many options supported here relate to footnotes. Footnotes are not specified by CommonMark, which we follow by default. They are supported by GitHub, so footnotes can be enabled in markdown with mdast-util-gfm.

The options footnoteBackLabel and footnoteLabel define natural language that explains footnotes, which is hidden for sighted users but shown to assistive technology. When your page is not in English, you must define translated values.

Back references use ARIA attributes, but the section label itself uses a heading that is hidden with an sr-only class. To show it to sighted users, define different attributes in footnoteLabelProperties.

Clobbering

Footnotes introduces a problem, as it links footnote calls to footnote definitions on the page through id attributes generated from user content, which results in DOM clobbering.

DOM clobbering is this:

<p id=x></p>
<script>alert(x) // `x` now refers to the DOM `p#x` element</script>

Elements by their ID are made available by browsers on the window object, which is a security risk. Using a prefix solves this problem.

More information on how to handle clobbering and the prefix is explained in Example: headings (DOM clobbering) in rehype-sanitize.

Unknown nodes

Unknown nodes are nodes with a type that isn’t in handlers or passThrough. The default behavior for unknown nodes is:

• when the node has a value (and doesn’t have data.hName, data.hProperties, or data.hChildren, see later), create a hast text node
• otherwise, create a <div> element (which could be changed with data.hName), with its children mapped from mdast to hast as well

This behavior can be changed by passing an unknownHandler.

FootnoteBackContentTemplate

Generate content for the backreference dynamically.

For the following markdown:

Alpha[^micromark], bravo[^micromark], and charlie[^remark].

[^remark]: things about remark
[^micromark]: things about micromark

This function will be called with:

• 0 and 0 for the backreference from things about micromark to alpha, as it is the first used definition, and the first call to it
• 0 and 1 for the backreference from things about micromark to bravo, as it is the first used definition, and the second call to it
• 1 and 0 for the backreference from things about remark to charlie, as it is the second used definition

Parameters

• referenceIndex (number) — index of the definition in the order that they are first referenced, 0-indexed
• rereferenceIndex (number) — index of calls to the same definition, 0-indexed

Returns

Content for the backreference when linking back from definitions to their reference (Array<ElementContent>, ElementContent, or string).

FootnoteBackLabelTemplate

Generate a back label dynamically.

For the following markdown:

Alpha[^micromark], bravo[^micromark], and charlie[^remark].

[^remark]: things about remark
[^micromark]: things about micromark

This function will be called with:

• 0 and 0 for the backreference from things about micromark to alpha, as it is the first used definition, and the first call to it
• 0 and 1 for the backreference from things about micromark to bravo, as it is the first used definition, and the second call to it
• 1 and 0 for the backreference from things about remark to charlie, as it is the second used definition

Parameters

• referenceIndex (number) — index of the definition in the order that they are first referenced, 0-indexed
• rereferenceIndex (number) — index of calls to the same definition, 0-indexed

Returns

Back label to use when linking back from definitions to their reference (string).

Handler

Handle a node (TypeScript type).

Parameters

• state (State) — info passed around
• node (MdastNode) — node to handle
• parent (MdastNode | undefined) — parent of node

Returns

Result (Array<HastNode> | HastNode | undefined).

Handlers

Handle nodes (TypeScript type).

Type

type Handlers = Partial<Record<Nodes['type'], Handler>>

Options

Configuration (TypeScript type).

Fields

• allowDangerousHtml (boolean, default: false) — whether to persist raw HTML in markdown in the hast tree
• clobberPrefix (string, default: 'user-content-') — prefix to use before the id property on footnotes to prevent them from clobbering
• file (VFile, optional) — corresponding virtual file representing the input document
• footnoteBackContent (FootnoteBackContentTemplate or string, default: defaultFootnoteBackContent) — content of the backreference back to references
• footnoteBackLabel (FootnoteBackLabelTemplate or string, default: defaultFootnoteBackLabel) — label to describe the backreference back to references
• footnoteLabel (string, default: 'Footnotes') — label to use for the footnotes section (affects screen readers)
• footnoteLabelProperties (Properties, default: {className: ['sr-only']}) — properties to use on the footnote label (note that id: 'footnote-label' is always added as footnote calls use it with aria-describedby to provide an accessible label)
• footnoteLabelTagName (string, default: h2) — tag name to use for the footnote label
• handlers (Handlers, optional) — extra handlers for nodes
• passThrough (Array<Nodes['type']>, optional) — list of custom mdast node types to pass through (keep) in hast (note that the node itself is passed, but eventual children are transformed)
• unknownHandler (Handler, optional) — handle all unknown nodes

Raw

Raw string of HTML embedded into HTML AST (TypeScript type).

Type

import type {Data, Literal} from 'hast'

interface Raw extends Literal {
  type: 'raw'
  data?: RawData | undefined
}

interface RawData extends Data {}

State

Info passed around about the current state (TypeScript type).

Fields

• all ((node: MdastNode) => Array<HastNode>) — transform the children of an mdast parent to hast
• applyData (<Type extends HastNode>(from: MdastNode, to: Type) => Type | HastElement) — honor the data of from and maybe generate an element instead of to
• definitionById (Map<string, Definition>) — definitions by their uppercased identifier
• footnoteById (Map<string, FootnoteDefinition>) — footnote definitions by their uppercased identifier
• footnoteCounts (Map<string, number>) — counts for how often the same footnote was called
• footnoteOrder (Array<string>) — identifiers of order when footnote calls first appear in tree order
• handlers (Handlers) — applied node handlers
• one ((node: MdastNode, parent: MdastNode | undefined) => HastNode | Array<HastNode> | undefined) — transform an mdast node to hast
• options (Options) — configuration
• patch ((from: MdastNode, to: HastNode) => undefined)
• wrap (<Type extends HastNode>(nodes: Array<Type>, loose?: boolean) => Array<Type | HastText>) — wrap nodes with line endings between each node, adds initial/final line endings when loose

Examples

Example: supporting HTML in markdown naïvely

If you completely trust authors (or plugins) and want to allow them to HTML in markdown, and the last utility has an allowDangerousHtml option as well (such as hast-util-to-html) you can pass allowDangerousHtml to this utility (mdast-util-to-hast):

import {fromMarkdown} from 'mdast-util-from-markdown'
import {toHast} from 'mdast-util-to-hast'
import {toHtml} from 'hast-util-to-html'

const markdown = 'It <i>works</i>! <img onerror="alert(1)">'
const mdast = fromMarkdown(markdown)
const hast = toHast(mdast, {allowDangerousHtml: true})
const html = toHtml(hast, {allowDangerousHtml: true})

console.log(html)

…now running node example.js yields:

<p>It <i>works</i>! <img onerror="alert(1)"></p>

⚠️ Danger: observe that the XSS attack through the onerror attribute is still present.

Example: supporting HTML in markdown properly

If you do not trust the authors of the input markdown, or if you want to make sure that further utilities can see HTML embedded in markdown, use hast-util-raw. The following example passes allowDangerousHtml to this utility (mdast-util-to-hast), then turns the raw embedded HTML into proper HTML nodes (hast-util-raw), and finally sanitizes the HTML by only allowing safe things (hast-util-sanitize):

import {raw} from 'hast-util-raw'
import {sanitize} from 'hast-util-sanitize'
import {toHtml} from 'hast-util-to-html'
import {fromMarkdown} from 'mdast-util-from-markdown'
import {toHast} from 'mdast-util-to-hast'

const markdown = 'It <i>works</i>! <img onerror="alert(1)">'
const mdast = fromMarkdown(markdown)
const hast = raw(toHast(mdast, {allowDangerousHtml: true}))
const safeHast = sanitize(hast)
const html = toHtml(safeHast)

console.log(html)

…now running node example.js yields:

<p>It <i>works</i>! <img></p>

👉 Note: observe that the XSS attack through the onerror attribute is no longer present.

Example: footnotes in languages other than English

If you know that the markdown is authored in a language other than English, and you’re using micromark-extension-gfm and mdast-util-gfm to match how GitHub renders markdown, and you know that footnotes are (or can?) be used, you should translate the labels associated with them.

Let’s first set the stage:

import {toHtml} from 'hast-util-to-html'
import {gfm} from 'micromark-extension-gfm'
import {fromMarkdown} from 'mdast-util-from-markdown'
import {gfmFromMarkdown} from 'mdast-util-gfm'
import {toHast} from 'mdast-util-to-hast'

const markdown = 'Bonjour[^1]\n\n[^1]: Monde!'
const mdast = fromMarkdown(markdown, {
  extensions: [gfm()],
  mdastExtensions: [gfmFromMarkdown()]
})
const hast = toHast(mdast)
const html = toHtml(hast)

console.log(html)

…now running node example.js yields:

<p>Bonjour<sup><a href="#user-content-fn-1" id="user-content-fnref-1" data-footnote-ref aria-describedby="footnote-label">1</a></sup></p>
<section data-footnotes class="footnotes"><h2 class="sr-only" id="footnote-label">Footnotes</h2>
<ol>
<li id="user-content-fn-1">
<p>Monde! <a href="#user-content-fnref-1" data-footnote-backref="" aria-label="Back to reference 1" class="data-footnote-backref">↩</a></p>
</li>
</ol>
</section>

This is a mix of English and French that screen readers can’t handle nicely. Let’s say our program does know that the markdown is in French. In that case, it’s important to translate and define the labels relating to footnotes so that screen reader users can properly pronounce the page:

@@ -9,7 +9,16 @@ const mdast = fromMarkdown(markdown, {
   extensions: [gfm()],
   mdastExtensions: [gfmFromMarkdown()]
 })
-const hast = toHast(mdast)
+const hast = toHast(mdast, {
+  footnoteLabel: 'Notes de bas de page',
+  footnoteBackLabel(referenceIndex, rereferenceIndex) {
+    return (
+      'Retour à la référence ' +
+      (referenceIndex + 1) +
+      (rereferenceIndex > 1 ? '-' + rereferenceIndex : '')
+    )
+  }
+})
 const html = toHtml(hast)

 console.log(html)

…now running node example.js with the above patch applied yields:

@@ -1,8 +1,8 @@
 <p>Bonjour<sup><a href="#user-content-fn-1" id="user-content-fnref-1" data-footnote-ref aria-describedby="footnote-label">1</a></sup></p>
-<section data-footnotes class="footnotes"><h2 class="sr-only" id="footnote-label">Footnotes</h2>
+<section data-footnotes class="footnotes"><h2 class="sr-only" id="footnote-label">Notes de bas de page</h2>
 <ol>
 <li id="user-content-fn-1">
-<p>Monde! <a href="#user-content-fnref-1" data-footnote-backref="" aria-label="Back to reference 1" class="data-footnote-backref">↩</a></p>
+<p>Monde! <a href="#user-content-fnref-1" data-footnote-backref="" aria-label="Retour à la référence 1" class="data-footnote-backref">↩</a></p>
 </li>
 </ol>
 </section>

Example: supporting custom nodes

This project supports CommonMark and the GFM constructs (footnotes, strikethrough, tables) and the frontmatter constructs YAML and TOML. Support can be extended to other constructs in two ways: a) with handlers, b) through fields on nodes.

For example, when we represent a mark element in markdown and want to turn it into a <mark> element in HTML, we can use a handler:

import {toHtml} from 'hast-util-to-html'
import {toHast} from 'mdast-util-to-hast'

const mdast = {
  type: 'paragraph',
  children: [{type: 'mark', children: [{type: 'text', value: 'x'}]}]
}

const hast = toHast(mdast, {
  handlers: {
    mark(state, node) {
      return {
        type: 'element',
        tagName: 'mark',
        properties: {},
        children: state.all(node)
      }
    }
  }
})

console.log(toHtml(hast))

We can do the same through certain fields on nodes:

import {toHtml} from 'hast-util-to-html'
import {toHast} from 'mdast-util-to-hast'

const mdast = {
  type: 'paragraph',
  children: [
    {
      type: 'mark',
      children: [{type: 'text', value: 'x'}],
      data: {hName: 'mark'}
    }
  ]
}

console.log(toHtml(toHast(mdast)))

Algorithm

This project by default handles CommonMark, GFM (footnotes, strikethrough, tables) and common frontmatter (YAML, TOML).

Existing handlers can be overwritten and handlers for more nodes can be added. It’s also possible to define how mdast is turned into hast through fields on nodes.

Default handling

The following table gives insight into what input turns into what output:

> A greater than…

<blockquote>
<p>A greater than…</p>
</blockquote>

A backslash\
before a line break…

<p>A backslash<br>
before a line break…</p>

```js
backtick.fences('for blocks')
```

<pre><code className="language-js">backtick.fences('for blocks')
</code></pre>

Two ~~tildes~~ for delete.

<p>Two <del>tildes</del> for delete.</p>

Some *asterisks* for emphasis.

<p>Some <em>asterisks</em> for emphasis.</p>

With a [^caret].

[^caret]: Stuff

<p>With a <sup><a href="#fn-caret" …>1</a></sup>.</p>…

# One number sign…
###### Six number signs…

<h1>One number sign…</h1>
<h6>Six number signs…</h6>

<kbd>CMD+S</kbd>

![Alt text](/logo.png "title")

<p><img src="/logo.png" alt="Alt text" title="title"></p>

![Alt text][logo]

[logo]: /logo.png "title"

<p><img src="/logo.png" alt="Alt text" title="title"></p>

Some `backticks` for inline code.

<p>Some <code>backticks</code> for inline code.</p>

[Example](https://example.com "title")

<p><a href="https://example.com" title="title">Example</a></p>

[Example][]

[example]: https://example.com "title"

<p><a href="https://example.com" title="title">Example</a></p>

* asterisks for unordered items

1. decimals and a dot for ordered items

<ul>
<li>asterisks for unordered items</li>
</ul>
<ol>
<li>decimals and a dot for ordered items</li>
</ol>

Just some text…

<p>Just some text…</p>

Anything!

<p>Anything!</p>

Two **asterisks** for strong.

<p>Two <strong>asterisks</strong> for strong.</p>

Anything!

<p>Anything!</p>

| Pipes |
| ----- |

<table>
<thead>
<tr>
<th>Pipes</th>
</tr>
</thead>
</table>

Three asterisks for a thematic break:

***

<p>Three asterisks for a thematic break:</p>
<hr>

+++
fenced = true
+++

---
fenced: yes
---

👉 Note: GFM prescribes that the obsolete align attribute on td and th elements is used. To use style attributes instead of obsolete features, combine this utility with @mapbox/hast-util-table-cell-style.

🧑‍🏫 Info: this project is concerned with turning one syntax tree into another. It does not deal with markdown syntax or HTML syntax. The preceding examples are illustrative rather than authoritative or exhaustive.

Fields on nodes

A frequent problem arises when having to turn one syntax tree into another. As the original tree (in this case, mdast for markdown) is in some cases limited compared to the destination (in this case, hast for HTML) tree, is it possible to provide more info in the original to define what the result will be in the destination? This is possible by defining data on mdast nodes, which this utility will read as instructions on what hast nodes to create.

An example is math, which is a nonstandard markdown extension, that this utility doesn’t understand. To solve this, mdast-util-math defines instructions on mdast nodes that this plugin does understand because they define a certain hast structure.

The following fields can be used:

• node.data.hName — define the element’s tag name
• node.data.hProperties — define extra properties to use
• node.data.hChildren — define hast children to use

hName

node.data.hName sets the tag name of an element. The following mdast:

{
  type: 'strong',
  data: {hName: 'b'},
  children: [{type: 'text', value: 'Alpha'}]
}

…yields (hast):

{
  type: 'element',
  tagName: 'b',
  properties: {},
  children: [{type: 'text', value: 'Alpha'}]
}

hProperties

node.data.hProperties sets the properties of an element. The following mdast:

{
  type: 'image',
  src: 'circle.svg',
  alt: 'Big red circle on a black background',
  data: {hProperties: {className: ['responsive']}}
}

…yields (hast):

{
  type: 'element',
  tagName: 'img',
  properties: {
    src: 'circle.svg',
    alt: 'Big red circle on a black background',
    className: ['responsive']
  },
  children: []
}

hChildren

node.data.hChildren sets the children of an element. The following mdast:

{
  type: 'code',
  lang: 'js',
  data: {
    hChildren: [
      {
        type: 'element',
        tagName: 'span',
        properties: {className: ['hljs-meta']},
        children: [{type: 'text', value: '"use strict"'}]
      },
      {type: 'text', value: ';'}
    ]
  },
  value: '"use strict";'
}

…yields (hast):

{
  type: 'element',
  tagName: 'pre',
  properties: {},
  children: [{
    type: 'element',
    tagName: 'code',
    properties: {className: ['language-js']},
    children: [
      {
        type: 'element',
        tagName: 'span',
        properties: {className: ['hljs-meta']},
        children: [{type: 'text', value: '"use strict"'}]
      },
      {type: 'text', value: ';'}
    ]
  }]
}

👉 Note: the pre and language-js class are normal mdast-util-to-hast functionality.

CSS

Assuming you know how to use (semantic) HTML and CSS, then it should generally be straightforward to style the HTML produced by this plugin. With CSS, you can get creative and style the results as you please.

Some semistandard features, notably GFMs tasklists and footnotes, generate HTML that be unintuitive, as it matches exactly what GitHub produces for their website. There is a project, sindresorhus/github-markdown-css, that exposes the stylesheet that GitHub uses for rendered markdown, which might either be inspirational for more complex features, or can be used as-is to exactly match how GitHub styles rendered markdown.

The following CSS is needed to make footnotes look a bit like GitHub:

/* Style the footnotes section. */
.footnotes {
  font-size: smaller;
  color: #8b949e;
  border-top: 1px solid #30363d;
}

/* Hide the section label for visual users. */
.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  word-wrap: normal;
  border: 0;
}

/* Place `[` and `]` around footnote calls. */
[data-footnote-ref]::before {
  content: '[';
}

[data-footnote-ref]::after {
  content: ']';
}

Syntax tree

The following interfaces are added to hast by this utility.

Nodes

Raw

interface Raw <: Literal {
  type: 'raw'
}

Raw (Literal) represents a string if raw HTML inside hast. Raw nodes are typically ignored but are handled by hast-util-to-html and hast-util-raw.

Types

This package is fully typed with TypeScript. It exports the FootnoteBackContentTemplate, FootnoteBackLabelTemplate, Handler, Handlers, Options, Raw, and State types.

It also registers the Raw node type with @types/hast. If you’re working with the syntax tree (and you pass allowDangerousHtml: true), make sure to import this utility somewhere in your types, as that registers the new node type in the tree.

/**
 * @typedef {import('mdast-util-to-hast')}
 */

import {visit} from 'unist-util-visit'

/** @type {import('hast').Root} */
const tree = { /* … */ }

visit(tree, function (node) {
  // `node` can now be `raw`.
})

Finally, it also registers the hChildren, hName, and hProperties fields on Data of @types/mdast. If you’re working with the syntax tree, make sure to import this utility somewhere in your types, as that registers the data fields in the tree.

/**
 * @typedef {import('mdast-util-to-hast')}
 */

import {visit} from 'unist-util-visit'

/** @type {import('hast').Root} */
const tree = { /* … */ }

console.log(tree.data?.hName) // Types as `string | undefined`.

Compatibility

Projects maintained by the unified collective are compatible with maintained versions of Node.js.

When we cut a new major release, we drop support for unmaintained versions of Node. This means we try to keep the current release line, mdast-util-to-hast@^13, compatible with Node.js 16.

Security

Use of mdast-util-to-hast can open you up to a cross-site scripting (XSS) attack. Embedded hast properties (hName, hProperties, hChildren), custom handlers, and the allowDangerousHtml option all provide openings.

The following example shows how a script is injected where a benign code block is expected with embedded hast properties:

const code = {type: 'code', value: 'alert(1)'}

code.data = {hName: 'script'}

Yields:

<script>alert(1)</script>

The following example shows how an image is changed to fail loading and therefore run code in a browser.

const image = {type: 'image', url: 'existing.png'}

image.data = {hProperties: {src: 'missing', onError: 'alert(2)'}}

Yields:

<img src="missing" onerror="alert(2)">

The following example shows the default handling of embedded HTML:

# Hello

<script>alert(3)</script>

Yields:

<h1>Hello</h1>

Passing allowDangerousHtml: true to mdast-util-to-hast is typically still not enough to run unsafe code:

<h1>Hello</h1>
&#x3C;script>alert(3)&#x3C;/script>

If allowDangerousHtml: true is also given to hast-util-to-html (or rehype-stringify), the unsafe code runs:

<h1>Hello</h1>
<script>alert(3)</script>

Use hast-util-sanitize to make the hast tree safe.

Related

• hast-util-to-mdast — transform hast to mdast
• hast-util-to-xast — transform hast to xast
• hast-util-sanitize — sanitize hast nodes

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