| .. | ||
| lib | ||
| index.d.ts | ||
| index.js | ||
| license | ||
| package.json | ||
| readme.md | ||
mdast-util-to-hast
mdast utility to transform to hast.
Contents
- What is this?
- When should I use this?
- Install
- Use
- API
- Examples
- Algorithm
- CSS
- Syntax tree
- 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-indexedrereferenceIndex(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-indexedrereferenceIndex(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
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-htmlalso has an optionallowDangerousHtmlwhich will output the raw HTML. This is typically discouraged as noted by the option name but is useful if you completely trust authorshast-util-rawcan 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 havedata.hName,data.hProperties, ordata.hChildren, see later), create a hasttextnode - otherwise, create a
<div>element (which could be changed withdata.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:
0and0for the backreference fromthings about micromarktoalpha, as it is the first used definition, and the first call to it0and1for the backreference fromthings about micromarktobravo, as it is the first used definition, and the second call to it1and0for the backreference fromthings about remarktocharlie, as it is the second used definition
Parameters
referenceIndex(number) — index of the definition in the order that they are first referenced, 0-indexedrereferenceIndex(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:
0and0for the backreference fromthings about micromarktoalpha, as it is the first used definition, and the first call to it0and1for the backreference fromthings about micromarktobravo, as it is the first used definition, and the second call to it1and0for the backreference fromthings about remarktocharlie, as it is the second used definition
Parameters
referenceIndex(number) — index of the definition in the order that they are first referenced, 0-indexedrereferenceIndex(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 aroundnode(MdastNode) — node to handleparent(MdastNode | undefined) — parent ofnode
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 treeclobberPrefix(string, default:'user-content-') — prefix to use before theidproperty on footnotes to prevent them from clobberingfile(VFile, optional) — corresponding virtual file representing the input documentfootnoteBackContent(FootnoteBackContentTemplateorstring, default:defaultFootnoteBackContent) — content of the backreference back to referencesfootnoteBackLabel(FootnoteBackLabelTemplateorstring, default:defaultFootnoteBackLabel) — label to describe the backreference back to referencesfootnoteLabel(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 thatid: 'footnote-label'is always added as footnote calls use it witharia-describedbyto provide an accessible label)footnoteLabelTagName(string, default:h2) — tag name to use for the footnote labelhandlers(Handlers, optional) — extra handlers for nodespassThrough(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 hastapplyData(<Type extends HastNode>(from: MdastNode, to: Type) => Type | HastElement) — honor thedataoffromand maybe generate an element instead oftodefinitionById(Map<string, Definition>) — definitions by their uppercased identifierfootnoteById(Map<string, FootnoteDefinition>) — footnote definitions by their uppercased identifierfootnoteCounts(Map<string, number>) — counts for how often the same footnote was calledfootnoteOrder(Array<string>) — identifiers of order when footnote calls first appear in tree orderhandlers(Handlers) — applied node handlersone((node: MdastNode, parent: MdastNode | undefined) => HastNode | Array<HastNode> | undefined) — transform an mdast node to hastoptions(Options) — configurationpatch((from: MdastNode, to: HastNode) => undefined)wrap(<Type extends HastNode>(nodes: Array<Type>, loose?: boolean) => Array<Type | HastText>) — wrapnodeswith line endings between each node, adds initial/final line endings whenloose
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
onerrorattribute 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
onerrorattribute 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:
| mdast node | markdown example | hast node | html example |
|---|---|---|---|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Nothing (default), |
n/a |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
||
|
|
|
|
|
|
||
|
|
|
|
|
|
|
|
|
|
|
Nothing |
n/a |
|
|
|
Nothing |
n/a |
👉 Note: GFM prescribes that the obsolete
alignattribute ontdandthelements is used. To usestyleattributes 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 namenode.data.hProperties— define extra properties to usenode.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
preandlanguage-jsclass are normalmdast-util-to-hastfunctionality.
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>
<script>alert(3)</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 mdasthast-util-to-xast— transform hast to xasthast-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.