mdast-util-to-hast

What is mdast-util-to-hast?

The mdast-util-to-hast package is a utility that allows for the transformation of MDAST (Markdown Abstract Syntax Tree) to HAST (Hypertext Abstract Syntax Tree). This is particularly useful for applications that need to convert markdown content into HTML or other formats that can be more easily manipulated or displayed in web environments.

What are mdast-util-to-hast's main functionalities?

Convert MDAST to HAST

This feature allows for the conversion of a markdown document represented as an MDAST into a HAST. The code sample demonstrates how to use unified with the remark-parse plugin to parse markdown into MDAST, and then convert that MDAST into HAST using mdast-util-to-hast.

const unified = require('unified')
const markdown = require('remark-parse')
const toHAST = require('mdast-util-to-hast')

unified()
  .use(markdown)
  .use(() => tree => toHAST(tree))
  .process('# Hello world!', function (err, file) {
    if (err) throw err
    console.log(file)
  })

Other packages similar to mdast-util-to-hast

remark-html

remark-html is a plugin for the remark processor that compiles markdown to HTML. It is similar to mdast-util-to-hast in that it also deals with the conversion of markdown content to a more web-friendly format. However, remark-html is more of an end-to-end solution for converting markdown directly to HTML, whereas mdast-util-to-hast provides a lower-level conversion to HAST, which can then be further manipulated or converted to HTML.

rehype

rehype is a processor powered by plugins part of the unified ecosystem that manipulates HTML documents. It is similar to mdast-util-to-hast in the sense that both deal with HAST nodes. However, rehype operates directly on HTML content or HAST, focusing on the manipulation and processing of HTML, whereas mdast-util-to-hast is specifically designed for converting MDAST to HAST.

README

mdast-util-to-hast

mdast utility to transform to hast.

Note: You probably want to use remark-rehype.

Install

This package is ESM only: Node 12+ is needed to use it and it must be imported instead of required.

npm:

npm install mdast-util-to-hast

Use

Say we have the following example.md:

## Hello **World**!

…and next to it, example.js:

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

const mdast = fromMarkdown(fs.readFileSync('example.md'))
const hast = toHast(mdast)
const html = toHtml(hast)

console.log(html)

Which when running with node example yields:

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

API

This package exports the following identifiers: toHast, defaultHandlers, all, one There is no default export.

toHast(node[, options])

Transform the given mdast tree to a hast tree.

Options

options.allowDangerousHtml

Whether to allow html nodes and inject them as raw HTML (boolean, default: false). Only do this when using hast-util-to-html (rehype-stringify) or hast-util-raw (rehype-raw) later: raw nodes are not a standard part of hast.

options.clobberPrefix

Prefix to use before the id attribute on footnotes to prevent it from clobbering (string, default: 'user-content-'). DOM clobbering is this:

<p id=x></p>
<script>alert(x)</script>

Elements by their ID are made available in browsers on the window object. Using a prefix this that from being a problem.

options.footnoteLabel

Label to use for the footnotes section (string, default: 'Footnotes'). Affects screen reader users. Change it if you’re authoring in a different language.

options.footnoteBackLabel

Label to use from backreferences back to their footnote call (string, default: 'Back to content'). Affects screen reader users. Change it if you’re authoring in a different language.

options.handlers

Object mapping mdast nodes to functions handling them. Take a look at lib/handlers/ for examples.

options.passThrough

List of custom mdast node types to pass through (keep) in hast (Array<string>, default: []). If the passed through nodes have children, those children are expected to be mdast and will be handled.

options.unknownHandler

Handler for unknown nodes (that aren’t in handlers or passThrough).

Default behavior:

• Unknown nodes with children are transformed to div elements
• Unknown nodes with value are transformed to text nodes

Returns

HastNode.

Notes

• yaml and toml nodes are ignored (created by remark-frontmatter)
• html nodes are ignored if allowDangerousHtml is false
• positions are properly patched
• node.data.hName configures the hast element’s tag-name
• node.data.hProperties is mixed into the hast element’s properties
• node.data.hChildren configures the hast element’s children
• GFM (and this project) uses the obsolete align attribute on td and th elements; combine this utility with @mapbox/hast-util-table-cell-style to use style instead

Examples

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, in hast:

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

hProperties

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

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

Yields, in 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, in hast (note: the pre and language-js class are normal mdast-util-to-hast functionality):

{
  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: ';'}
    ]
  }]
}

defaultHandlers

Object mapping mdast node types to functions that can handle them. See lib/handlers/index.js.

all(h, parent)

Helper function for writing custom handlers passed to options.handlers. Pass it h and a parent node (mdast) and it will turn the node’s children into an array of transformed nodes (hast).

one(h, node, parent)

Helper function for writing custom handlers passed to options.handlers. Pass it h, a node, and its parent (mdast) and it will turn node into hast content.

Recommended CSS

The following CSS is needed to make footnotes look a bit like GitHub. For the complete actual CSS that GitHub uses see sindresorhus/github-markdown-css.

/* 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: ']';
}

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:

var 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.

var 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

• mdast-util-to-nlcst — transform mdast to nlcst
• hast-util-to-mdast — transform hast to mdast
• hast-util-to-xast — transform hast to xast
• hast-util-sanitize — sanitize hast nodes
• remark-rehype — rehype support for remark
• rehype-remark — remark support for rehype

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