Project: micromark/micromark-extension-gfm-footnote

Package: micromark-extension-gfm-footnote@1.0.4

  1. Dependents: 0
  2. micromark extension to support GFM footnotes
  1. unified 181
  2. markdown 154
  3. micromark 37
  4. gfm 21
  5. micromark-extension 17
  6. definition 16
  7. footnote 6
  8. note 4


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micromark extensions to support GFM footnotes.


What is this?

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

GitHub announced footnotes on September 30, 2021 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. 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. To support all GFM features, use micromark-extension-gfm instead.

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

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


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

npm install micromark-extension-gfm-footnote

In Deno with esm.sh:

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

In browsers with esm.sh:

<script type="module">
  import {gfmFootnote, gfmFootnoteHtml} from 'https://esm.sh/micromark-extension-gfm-footnote@2?bundle'


Say our document example.md contains:

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](https://docs.github.com/en/github/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#footnotes)

…and our module example.js looks as follows:

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()]


…now running node example.js yields:

<p>Using footnotes is fun!<sup><a href="#user-content-fn-1" id="user-content-fnref-1" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup> They let you reference relevant information without disrupting the flow of what you’re trying to say.<sup><a href="#user-content-fn-bignote" id="user-content-fnref-bignote" data-footnote-ref="" aria-describedby="footnote-label">2</a></sup></p>
<p>Text here and here and here.
<a href="https://docs.github.com/en/github/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#footnotes">Learn more about markdown and footnotes in markdown</a></p>
<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
<li id="user-content-fn-1">
<p>This is the first footnote. <a href="#user-content-fnref-1" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content"></a></p>
<li id="user-content-fn-bignote">
<p>Here’s one with multiple paragraphs and code.</p>
<p>Indent paragraphs to include them in the footnote.</p>
<pre><code>my code
<p>Add as many paragraphs as you like. <a href="#user-content-fnref-bignote" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content"></a></p>


This package exports the identifiers defaultBackLabel, gfmFootnote, and gfmFootnoteHtml. There is no default export.

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

defaultBackLabel(referenceIndex, rereferenceIndex)

Generate the default label that GitHub uses on backreferences (BackLabelTemplate).


Create an extension for micromark to enable GFM footnote syntax.


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


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


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


Generate a back label dynamically (TypeScript type).

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:


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


Configuration (TypeScript type).


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:

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

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.


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.


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.


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.


Textual label to describe the backreference back to footnote calls (BackLabelTemplate or string, default: defaultBackLabel).

Change it when the markdown is not in English.

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


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:


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.


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 <sup> and <a> elements in HTML:

<sup><a href="#user-content-fn-x" id="user-content-fnref-x" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup></p>

…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, § 4.5.1 The a element, and § Embedding custom non-visible data with the data-* attributes in the HTML spec, and § 6.8 aria-describedby property 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 <section>, <h2>, and <ol> elements:

<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>

Each definition is generated as a <li> in the <ol> in the order they were first referenced:

<li id="user-content-fn-1"></li>

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 <span> after .

<a href="#user-content-fnref-1" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content"></a> <a href="#user-content-fnref-1-2" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content"><sup>2</sup></a>

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


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.

/* 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: ']';


Footnotes form with, roughly, the following 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 for more on parsing details.

The identifiers in the label parts are interpreted as the string 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. 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:

[^a]: x
[^a]: y


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

[^a&amp;b]: x
[^a\&b]: y


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


This package is fully typed with TypeScript. It exports the additional types BackLabelTemplate and HtmlOptions.


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.


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.


See contributing.md in micromark/.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.


MIT © Titus Wormer