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    mdast-util-to-hast
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    mdast-util-to-hast

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    mdast utility to transform to hast.

    Contents

    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 12.20+, 14.14+, or 16.0+), install with npm:

    npm install mdast-util-to-hast

    In Deno with esm.sh:

    import {toHast} from "https://esm.sh/mdast-util-to-hast@12"

    In browsers with esm.sh:

    <script type="module">
      import {toHast} from "https://esm.sh/mdast-util-to-hast@12?bundle"
    </script>

    Use

    Say we have the following example.md:

    ## Hello **World**!

    …and next to it a module example.js:

    import {promises as 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'
    
    main()
    
    async function main() {
      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 toHast, defaultHandlers, all, and one. There is no default export.

    toHast(node[, options])

    mdast utility to transform to hast.

    options

    Configuration (optional).

    options.allowDangerousHtml

    Whether to persist raw HTML in markdown in the hast tree (boolean, default: false). 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
    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) // `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.

    👉 Note: this option affects footnotes. Footnotes are not specified by CommonMark. They are supported by GitHub, so they can be enabled by using the utility mdast-util-gfm.

    options.footnoteLabel

    Label to use for the footnotes section (string, default: 'Footnotes'). Affects screen readers. Change it when the markdown is not in English.

    👉 Note: this option affects footnotes. Footnotes are not specified by CommonMark. They are supported by GitHub, so they can be enabled by using the utility mdast-util-gfm.

    options.footnoteBackLabel

    Label to use from backreferences back to their footnote call (string, default: 'Back to content'). Affects screen readers. Change it when the markdown is not in English.

    👉 Note: this option affects footnotes. Footnotes are not specified by CommonMark. They are supported by GitHub, so they can be enabled by using the utility mdast-util-gfm.

    options.handlers

    Object mapping node types to functions handling the corresponding nodes. See lib/handlers/ for examples.

    In a handler, you have access to h, which should be used to create hast nodes from mdast nodes. On h, there are several fields that may be of interest.

    options.passThrough

    List of 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.

    Similar functionality can be achieved with a custom handler. A passThrough of ['customNode'] is equivalent to:

    toHast(/* … */, {
      handlers: {
        customNode(h, node) {
          return 'children' in node ? {...node, children: all(h, node)} : node
        }
      }
    })
    options.unknownHandler

    Handler for unknown nodes (Handler?). 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
    Returns

    HastNode.

    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.

    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 {fromMarkdown} from 'mdast-util-from-markdown'
    import {toHast} from 'mdast-util-to-hast'
    import {raw} from 'hast-util-raw'
    import {sanitize} from 'hast-util-sanitize'
    import {toHtml} from 'hast-util-to-html'
    
    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 {fromMarkdown} from 'mdast-util-from-markdown'
    import {gfm} from 'micromark-extension-gfm'
    import {gfmFromMarkdown} from 'mdast-util-gfm'
    import {toHast} from 'mdast-util-to-hast'
    import {toHtml} from 'hast-util-to-html'
    
    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 id="footnote-label" class="sr-only">Footnotes</h2>
    <ol>
    <li id="user-content-fn-1">
    <p>Monde! <a href="#user-content-fnref-1" data-footnote-backref class="data-footnote-backref" aria-label="Back to content"></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 +9,10 @@ const mdast = fromMarkdown(markdown, {
       extensions: [gfm()],
       mdastExtensions: [gfmFromMarkdown()]
     })
    -const hast = toHast(mdast)
    +const hast = toHast(mdast, {
    +  footnoteLabel: 'Notes de bas de page',
    +  footnoteBackLabel: 'Arrière'
    +})
     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 id="footnote-label" class="sr-only">Footnotes</h2>
    +<section data-footnotes class="footnotes"><h2 id="footnote-label" class="sr-only">Notes de bas de page</h2>
     <ol>
     <li id="user-content-fn-1">
    -<p>Monde! <a href="#user-content-fnref-1" data-footnote-backref class="data-footnote-backref" aria-label="Back to content">↩</a></p>
    +<p>Monde! <a href="#user-content-fnref-1" data-footnote-backref class="data-footnote-backref" aria-label="Arrière">↩</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 {toHast, all} from 'mdast-util-to-hast'
    import {toHtml} from 'hast-util-to-html'
    
    const mdast = {
      type: 'paragraph',
      children: [{type: 'mark', children: [{type: 'text', value: 'x'}]}]
    }
    
    const hast = toHast(mdast, {
      handlers: {
        mark(h, node) {
          return h(node, 'mark', all(h, node))
        }
      }
    })
    
    console.log(toHtml(hast))

    We can do the same through certain fields on nodes:

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

    blockquote

    > A greater than…

    element (blockquote)

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

    break

    A backslash\
    before a line break…

    element (br)

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

    code

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

    element (pre and code)

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

    delete (GFM)

    Two ~~tildes~~ for delete.

    element (del)

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

    emphasis

    Some *asterisks* for emphasis.

    element (em)

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

    footnoteReference, footnoteDefinition (GFM)

    With a [^caret].
    
    [^caret]: Stuff

    element (section, sup, a)

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

    heading

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

    element (h1h6)

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

    html

    <kbd>CMD+S</kbd>

    Nothing (default), raw (when allowDangerousHtml: true)

    n/a

    image

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

    element (img)

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

    imageReference, definition

    ![Alt text][logo]
    
    [logo]: /logo.png "title"

    element (img)

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

    inlineCode

    Some `backticks` for inline code.

    element (code)

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

    link

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

    element (a)

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

    linkReference, definition

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

    element (a)

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

    list, listItem

    * asterisks for unordered items
    
    1. decimals and a dot for ordered items

    element (li and ol or ul)

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

    paragraph

    Just some text…

    element (p)

    <p>Just some text…</p>

    root

    Anything!

    root

    <p>Anything!</p>

    strong

    Two **asterisks** for strong.

    element (strong)

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

    text

    Anything!

    text

    <p>Anything!</p>

    table, tableRow, tableCell

    | Pipes |
    | ----- |

    element (table, thead, tbody, tr, td, th)

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

    thematicBreak

    Three asterisks for a thematic break:
    
    ***

    element (hr)

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

    toml (frontmatter)

    +++
    fenced = true
    +++

    Nothing

    n/a

    yaml (frontmatter)

    ---
    fenced: yes
    ---

    Nothing

    n/a

    👉 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 configures the element’s tag name
    • node.data.hProperties is mixed into the element’s properties
    • node.data.hChildren configures the element’s children
    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',
      title: null,
      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 straight forward 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 also exports Options, Handler, Handlers, H, and Raw types.

    If you’re working with raw nodes in the hast syntax tree (which are added when allowDangerousHtml: true), make sure to import this utility somewhere in your types, as that registers the new node types in the tree.

    /** @typedef {import('mdast-util-to-hast')} */
    import {visit} from 'unist-util-visit'
    
    /** @type {import('hast').Root} */
    const tree = { /* … */ }
    
    visit(tree, (node) => {
      // `node` can now be `raw`.
    })

    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.

    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

    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

    Install

    npm i mdast-util-to-hast

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    Version

    12.1.2

    License

    MIT

    Unpacked Size

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    Total Files

    62

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    • wooorm
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