micromark-extension-gfm-footnote

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[micromark][] extensions to support GFM [footnotes][post].

Contents

* What is this?
* When to use this
* Install
* Use
* API
* defaultBackLabel(referenceIndex, rereferenceIndex)
* gfmFootnote()
* gfmFootnoteHtml(options?)
* BackLabelTemplate
* HtmlOptions
* Bugs
* Authoring
* HTML
* CSS
* Syntax
* Types
* Compatibility
* Security
* Related
* Contribute
* License

What is this?

This package contains extensions that add support for footnotes as enabled by
GFM to [micromark][micromark].

GitHub announced footnotes [on September 30, 2021][post] but did not specify
them in their GFM spec.
As they are implemented in their parser and supported in all places where
other GFM features work, they can be considered part of GFM.
GitHub employs several other features (such as mentions or frontmatter) that
are either not in their parser, or not in all places where GFM features work,
which should not be considered GFM.

The implementation of footnotes on github.com is currently buggy.
The bugs have been reported on [cmark-gfm][cmark-gfm].
This micromark extension matches github.com except for its bugs.

When to use this

This project is useful when you want to support footnotes in markdown.

You can use these extensions when you are working with [micromark][micromark].
To support all GFM features, use
[micromark-extension-gfm][micromark-extension-gfm] instead.

When you need a syntax tree, combine this package with
[mdast-util-gfm-footnote][mdast-util-gfm-footnote].

All these packages are used in [remark-gfm][remark-gfm], which focusses on
making it easier to transform content by abstracting these internals away.

Install

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

``sh
npm install micromark-extension-gfm-footnote
`

In Deno with [esm.sh][esmsh]:

`js
import {gfmFootnote, gfmFootnoteHtml} from 'https://esm.sh/micromark-extension-gfm-footnote@2'
`

In browsers with [esm.sh][esmsh]:

`html

`

Use

Say our document example.md contains:

``markdown
Using footnotes is fun![^1] They let you reference relevant information without disrupting the flow of what you’re trying to say.[^bignote]

[^1]: This is the first footnote.
[^bignote]: Here’s one with multiple paragraphs and code.

Indent paragraphs to include them in the footnote.

`
my code
`

Add as many paragraphs as you like.

Text here and here and here.
Learn more about markdown and footnotes in markdown
``

…and our module example.js looks as follows:

`js
import fs from 'node:fs/promises'
import {micromark} from 'micromark'
import {gfmFootnote, gfmFootnoteHtml} from 'micromark-extension-gfm-footnote'

const output = micromark(await fs.readFile('example.md'), {
extensions: [gfmFootnote()],
htmlExtensions: [gfmFootnoteHtml()]
})

console.log(output)
`

…now running node example.js yields:

`html


Using footnotes is fun!1 They let you reference relevant information without disrupting the flow of what you’re trying to say.2

Text here and here and here.
Learn more about markdown and footnotes in markdown

Footnotes

1. 
   

   This is the first footnote. ↩

   
   


2. 
   

   Here’s one with multiple paragraphs and code.

   
   

   Indent paragraphs to include them in the footnote.

   
   

   my code
   

   
   

   Add as many paragraphs as you like. ↩

   
   

`

API

This package exports the identifiers
[defaultBackLabel][api-default-back-label],
[gfmFootnote][api-gfm-footnote], and
[gfmFootnoteHtml][api-gfm-footnote-html].
There is no default export.

The export map supports the [development condition][development].
Run node --conditions development module.js to get instrumented dev code.
Without this condition, production code is loaded.

$3

Generate the default label that GitHub uses on backreferences
([BackLabelTemplate][api-back-label-template]).

$3

Create an extension for micromark to enable GFM footnote syntax.

###### Returns

Extension for micromark that can be passed in extensions to enable GFM
footnote syntax ([Extension][micromark-extension]).

$3

Create an extension for micromark to support GFM footnotes when serializing
to HTML.

###### Parameters

* options ([HtmlOptions][api-html-options], optional)
— configuration

###### Returns

Extension for micromark that can be passed in htmlExtensions to support GFM
footnotes when serializing to HTML
([HtmlExtension][micromark-html-extension]).

$3

Generate a back label dynamically (TypeScript type).

For the following markdown:

`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).

$3

Configuration (TypeScript type).

##### Fields

###### clobberPrefix

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

Pass '' for trusted markdown and when you are careful with polyfilling.
You could pass a different prefix.

DOM clobbering is this:

`html




`

The above example shows that elements are made available by browsers, by their
ID, on the window object.
This is a security risk because you might be expecting some other variable at
that place.
It can also break polyfills.
Using a prefix solves these problems.

###### label

Textual label to use for the footnotes section (string, default:
'Footnotes').

Change it when the markdown is not in English.

This label is typically hidden visually (assuming a sr-only CSS class
is defined that does that) and so affects screen readers only.

###### labelAttributes

Attributes to use on the footnote label (string, default:
'class="sr-only"').

Change it to show the label and add other attributes.

This label is typically hidden visually (assuming an sr-only CSS class
is defined that does that) and so affects screen readers only.
If you do have such a class, but want to show this section to everyone,
pass an empty string.
You can also add different attributes.

> 👉 Note: id="footnote-label" is always added, because footnote
> calls use it with aria-describedby to provide an accessible label.

###### labelTagName

HTML tag name to use for the footnote label element (string, default:
'h2').

Change it to match your document structure.

This label is typically hidden visually (assuming a sr-only CSS class
is defined that does that) and so affects screen readers only.

###### backLabel

Textual label to describe the backreference back to footnote calls
([BackLabelTemplate][api-back-label-template] or string,
default: [defaultBackLabel][api-default-back-label]).

Change it when the markdown is not in English.

This label is used in the [aria-label][aria-label] attribute on each
backreference (the ↩ links).
It affects users of assistive technology.

Bugs

GitHub’s own algorithm to parse footnote definitions contains several bugs.
These are not present in this project.
The issues relating to footnote definitions are:

* Footnote reference call identifiers are trimmed, but definition
identifiers aren’t\
— initial and final whitespace in labels causes them not to match
* Footnotes are matched case-insensitive, but links keep their casing,
breaking them\
— using uppercase (or any character that will be percent encoded) in
identifiers breaks links
* Colons in footnotes generate links w/o
href\
— colons in identifiers generate broken links
* [Character escape of ] does not work in footnote
identifiers](https://github.com/github/cmark-gfm/issues/240)\
— some character escapes don’t work
* Footnotes in links are
broken\
— while CommonMark prevents links in links, GitHub does not prevent
footnotes (which turn into links) in links
* Footnote-like brackets around image, break that
image\
— images can’t be used in what looks like a footnote call
* GFM footnotes: line ending in footnote definition label causes text to
disappear\
— line endings in footnote definitions cause text to disappear

Authoring

When authoring markdown with footnotes it’s recommended to use words instead
of numbers (or letters or anything with an order) as identifiers.
That makes it easier to reuse and reorder footnotes.

It’s recommended to place footnotes definitions at the bottom of the document.

HTML

GFM footnotes do not, on their own, relate to anything in HTML.
When a footnote reference matches with a definition, they each relate to several
elements in HTML.

The reference relates to and elements in HTML:

`html
1


`

…where x is the identifier used in the markdown source and 1 the number of
corresponding, listed, definition.

See [§ 4.5.19 The sub and sup elements][html-sup],
[§ 4.5.1 The a element][html-a], and
[§ 3.2.6.6 Embedding custom non-visible data with the data-
attributes*][html-data]
in the HTML spec, and
[§ 6.8 aria-describedby property][aria-describedby]
in WAI-ARIA, for more info.

When one or more definitions are referenced, a footnote section is generated at
the end of the document, using

,

, and

elements:

`html


Footnotes




…




`

Each definition is generated as a

1. in the
   in the order they were
   first referenced:

   `html


   1. …
   
`

   Backreferences are injected at the end of the first paragraph, or, when there
is no paragraph, at the end of the definition.
When a definition is referenced multiple times, multiple backreferences are
generated.
Further backreferences use an extra counter in the href attribute and
visually in a after ↩.

   `html
↩ ↩2
`

   See
[§ 4.5.1 The a element][html-a],
[§ 4.3.6 The h1, h2, h3, h4, h5, and h6 elements][html-h],
[§ 4.4.8 The li element][html-li],
[§ 4.4.5 The ol element][html-ol],
[§ 4.4.1 The p element][html-p],
[§ 4.3.3 The section element][html-section], and
[§ 4.5.19 The sub and sup elements][html-sup]
in the HTML spec, and
[§ 6.8 aria-label property][aria-label]
in WAI-ARIA, for more info.

   CSS

   The following CSS is needed to make footnotes look a bit like GitHub (and fixes
a bug).
For the complete actual CSS see
sindresorhus/github-markdown-css.

   `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 references. /
[data-footnote-ref]::before {
content: '[';
}

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

   Syntax

   Footnotes form with, roughly, the following BNF:

   `bnf
gfm_footnote_reference ::= gfm_footnote_label

   gfm_footnote_definition_start ::= gfm_footnote_label ':' *space_or_tab
; Restriction: blank line allowed.
gfm_footnote_definition_cont ::= 4(space_or_tab)

   ; Restriction: maximum 999 codes between ^ and ].
gfm_footnote_label ::= '[' '^' 1*(gfm_footnote_label_byte | gfm_footnote_label_escape) ']'
gfm_footnote_label_byte ::= text - '[' - '\\' - ']'
gfm_footnote_label_escape ::= '\\' ['[' | '\\' | ']']

   ; Any byte (u8)
byte ::= 0x00..=0xFFFF
space_or_tab ::= '\t' | ' '
eol ::= '\n' | '\r' | '\r\n'
line ::= byte - eol
text ::= line - space_or_tab
`

   Further lines after gfm_footnote_definition_start that are not prefixed with
gfm_footnote_definition_cont cause the footnote definition to be exited,
except when those lines are lazy continuation or blank.
Like so many things in markdown, footnote definition too are complex.
See [§ Phase 1: block structure in CommonMark][commonmark-block] for more
on parsing details.

   The identifiers in the label parts are interpreted as the
[string][micromark-content-types] content type.
That means that character escapes and character references are allowed.

   Definitions match to references through identifiers.
To match, both labels must be equal after normalizing with
[normalizeIdentifier][micromark-normalize-identifier].
One definition can match to multiple calls.
Multiple definitions with the same, normalized, identifier are ignored: the
first definition is preferred.
To illustrate, the definition with the content of x wins:

   `markdown
[^a]: x
[^a]: y

   [^a]
`

   Importantly, while labels can include [string][micromark-content-types]
content (character escapes and character references), these are not considered
when matching.
To illustrate, neither definition matches the reference:

   `markdown
[^a&b]: x
[^a\&b]: y

   [^a&b]
`

   Because footnote definitions are containers (like block quotes and list items),
they can contain more footnote definitions.
They can even include references to themselves.

   Types

   This package is fully typed with [TypeScript][].
It exports the additional types [BackLabelTemplate][api-back-label-template]
and [HtmlOptions][api-html-options].

   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,
micromark-extension-gfm-footnote@^2, compatible with Node.js 16.

   This package works with micromark version 3 and later.

   Security

   This package is safe.
Setting clobberPrefix = '' is dangerous, it opens you up to DOM clobbering.
The labelTagName and labelAttributes options are unsafe when used with user
content, they allow defining arbitrary HTML.

   Related

   * [micromark-extension-gfm][micromark-extension-gfm]
— support all of GFM
* [mdast-util-gfm-footnote][mdast-util-gfm-footnote]
— support all of GFM in mdast
* [mdast-util-gfm][mdast-util-gfm]
— support all of GFM in mdast
* [remark-gfm][remark-gfm]
— support all of GFM in remark

   Contribute

   See [contributing.md in micromark/.github][contributing] for ways to get
started.
See [support.md`][support] for ways to get help.

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

   License

   [MIT][license] © [Titus Wormer][author]

   [build-badge]: https://github.com/micromark/micromark-extension-gfm-footnote/workflows/main/badge.svg

   [build]: https://github.com/micromark/micromark-extension-gfm-footnote/actions

   [coverage-badge]: https://img.shields.io/codecov/c/github/micromark/micromark-extension-gfm-footnote.svg

   [coverage]: https://codecov.io/github/micromark/micromark-extension-gfm-footnote

   [downloads-badge]: https://img.shields.io/npm/dm/micromark-extension-gfm-footnote.svg

   [downloads]: https://www.npmjs.com/package/micromark-extension-gfm-footnote

   [size-badge]: https://img.shields.io/badge/dynamic/json?label=minzipped%20size&query=$.size.compressedSize&url=https://deno.bundlejs.com/?q=micromark-extension-gfm-footnote

   [size]: https://bundlejs.com/?q=micromark-extension-gfm-footnote

   [sponsors-badge]: https://opencollective.com/unified/sponsors/badge.svg

   [backers-badge]: https://opencollective.com/unified/backers/badge.svg

   [collective]: https://opencollective.com/unified

   [chat-badge]: https://img.shields.io/badge/chat-discussions-success.svg

   [chat]: https://github.com/micromark/micromark/discussions

   [npm]: https://docs.npmjs.com/cli/install

   [esmsh]: https://esm.sh

   [license]: license

   [author]: https://wooorm.com

   [contributing]: https://github.com/micromark/.github/blob/main/contributing.md

   [support]: https://github.com/micromark/.github/blob/main/support.md

   [coc]: https://github.com/micromark/.github/blob/main/code-of-conduct.md

   [esm]: https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c

   [typescript]: https://www.typescriptlang.org

   [development]: https://nodejs.org/api/packages.html#packages_resolving_user_conditions

   [micromark]: https://github.com/micromark/micromark

   [micromark-content-types]: https://github.com/micromark/micromark#content-types

   [micromark-extension]: https://github.com/micromark/micromark#syntaxextension

   [micromark-html-extension]: https://github.com/micromark/micromark#htmlextension

   [micromark-normalize-identifier]: https://github.com/micromark/micromark/tree/main/packages/micromark-util-normalize-identifier

   [micromark-extension-gfm]: https://github.com/micromark/micromark-extension-gfm

   [mdast-util-gfm-footnote]: https://github.com/syntax-tree/mdast-util-gfm-footnote

   [mdast-util-gfm]: https://github.com/syntax-tree/mdast-util-gfm

   [remark-gfm]: https://github.com/remarkjs/remark-gfm

   [post]: https://github.blog/changelog/2021-09-30-footnotes-now-supported-in-markdown-fields/

   [cmark-gfm]: https://github.com/github/cmark-gfm

   [commonmark-block]: https://spec.commonmark.org/0.30/#phase-1-block-structure

   [html-a]: https://html.spec.whatwg.org/multipage/text-level-semantics.html#the-a-element

   [html-data]: https://html.spec.whatwg.org/multipage/dom.html#embedding-custom-non-visible-data-with-the-data-*-attributes

   [html-h]: https://html.spec.whatwg.org/multipage/sections.html#the-h1,-h2,-h3,-h4,-h5,-and-h6-elements

   [html-li]: https://html.spec.whatwg.org/multipage/grouping-content.html#the-li-element

   [html-ol]: https://html.spec.whatwg.org/multipage/grouping-content.html#the-ol-element

   [html-p]: https://html.spec.whatwg.org/multipage/grouping-content.html#the-p-element

   [html-section]: https://html.spec.whatwg.org/multipage/sections.html#the-section-element

   [html-sup]: https://html.spec.whatwg.org/multipage/text-level-semantics.html#the-sub-and-sup-elements

   [aria-describedby]: https://w3c.github.io/aria/#aria-describedby

   [aria-label]: https://w3c.github.io/aria/#aria-label

   [api-gfm-footnote]: #gfmfootnote

   [api-gfm-footnote-html]: #gfmfootnotehtmloptions

   [api-html-options]: #htmloptions

   [api-default-back-label]: #defaultbacklabelreferenceindex-rereferenceindex

   [api-back-label-template]: #backlabeltemplate