Webmentions in Eleventy

There comes a time in every Eleventy blog's life where the author must write a post about implementing webmentions. Here is mine.


Webmentions enables cross-site conversations across the web. Replies, likes, reposts – you name it.

Eleventy is everyone’s favorite static site generator.

Two great tastes that taste great together.

There is so much prior art it’s almost silly. In no particular order:

That is all to say… seems like a solved problem. I’m not breaking new ground with anything I’ve done here.

I will say that it seems like most posts are focused on receiving webmentions and I wish there were more resources on sending webmentions. More on that below.

Receiving webmentions

I set up 11ty to fetch my webmentions as a global data file:

// Lifted almost entirely from: https://github.com/cdransf/coryd.dev/blob/2c64737f1fd97514f791178e59d07ad861370fbe/src/_data/webmentions.js
// Any changes or bugs are mine!
const EleventyFetch = require('@11ty/eleventy-fetch');

module.exports = async function () {
  const webmentionKey = process.env.WEBMENTION_IO_TOKEN;
  const url = `https://webmention.io/api/mentions.jf2?token=${webmentionKey}&per-page=1000`;
  try {
    const webmentions = await EleventyFetch(url, {
      duration: '1h',
      type: 'json',
    return webmentions.children;
  } catch (e) {
    console.error('Error fetching webmentions', e);
    // Return a blank.
    return [];

I’m using the @11ty/eleventy-fetch package so my builds don’t yank on the webmention.io API too much during development; it caches responses. The WEBMENTION_IO_TOKEN is an environment variable that I have set in my local development so I can see webmentions in dev. It’s also set as an environment variable in Netlify when it builds my site. I received my own token after signing in to Webmention.io.

So now all my templates have a webmentions variable available with data for the whole site. That’s okay, but if I’m within the context of a single page I want to show webmentions for only that page.

What is webmention.io sending us?

Here’s the entire webmentions structure I received from webmention.io as of 2023-08-30. I only have a single interaction 😿 so far (I’ll be okay). When my templates access webmentions, this is what they see:

    "type": "entry",
    "author": {
      "type": "card",
      "name": "Webmention Rocks!",
      "photo": "https://webmention.io/avatar/webmention.rocks/e08155b03da96cb1bdfd161ea24efdfad8d85d06afcee540ec246f1f613eb5a9.png",
      "url": ""
    "url": "https://webmention.rocks/receive/1",
    "published": "2023-08-26T16:17:32",
    "wm-received": "2023-08-26T16:17:33Z",
    "wm-id": 1709428,
    "wm-source": "https://webmention.rocks/receive/1/f59a68e4d1fc23ca612411c9af3d93bb",
    "wm-target": "https://drhayes.io/writing/pretty-atom-feed/",
    "wm-protocol": "webmention",
    "name": "Receiver Test #1",
    "content": {
      "html": "<p>This test verifies that you accept a Webmention request that contains a valid source and target URL. To pass this test, your Webmention endpoint must return either HTTP 200, 201 or 202 along with the <a href=\"https://www.w3.org/TR/webmention/#receiving-webmentions\">appropriate headers</a>.</p>\n        <p>If your endpoint returns HTTP 201, then it MUST also return a <code>Location</code> header. If it returns HTTP 200 or 202, then it MUST NOT include a <code>Location</code> header.</p>",
      "text": "This test verifies that you accept a Webmention request that contains a valid source and target URL. To pass this test, your Webmention endpoint must return either HTTP 200, 201 or 202 along with the appropriate headers.\n        If your endpoint returns HTTP 201, then it MUST also return a Location header. If it returns HTTP 200 or 202, then it MUST NOT include a Location header."
    "mention-of": "https://drhayes.io/writing/pretty-atom-feed/",
    "wm-property": "mention-of",
    "wm-private": false

I received this by testing my implementation using Webmention Rocks!, in the “Testing your Receiver” section all the way at the bottom of the page. It sent a mention-of my way. Sort of.

Seeing this structure raised so many questions for me:

I couldn’t find answers to those questions in the webmentions spec but that seems to mostly be about sending, not receiving. I mean, it has a section about receiving webmentions but that doesn’t seem super related to what webmention.io is sending me.

Turns out I should’ve been looking at the README for the webmention.io project, duh! In addition to the properties I’ve seen it also includes bookmark-of (ooh!) and rsvp (ooooooh!). So that’s neato.

I’m calling the service with a token but it looks like I don’t need one in some cases. For instance: https://webmention.io/api/mentions.jf2?target[]=https://drhayes.io/writing/pretty-atom-feed/ will return the webmentions for that particular URL only. If I were working client-side that might be perfect – no exposed token that way. And it looks like you can receive data for multiple pages that way, so that’s something to keep in your back pocket.

For more answers about the type fields, I turned to http://microformats.org/wiki/jf2. It seems like the type params are the suffixes of the various microformat objects. The jf2 spec backs me up on that one. Hooray for reading comprehension!

I am super glad that Aaron Parecki is, apparently, running this for the good of the community. Since Netlify has edge functions, though, I wonder if I could get something running to handle my own webmentions just in case.

…and how do I integrate it?

I have this in an include named webmentions.njk:

{% if webmentions %}
{% set mentions = webmentions | webmentionsByUrl(aliases) %}
... template stuff ...
{% endif %}

Simply: if I have webmentions, do some stuff. But not just “webmentions”, but “webmentions for this page” using the webmentionsByUrl filter:

function webmentionsByUrl(webmentions, aliases) {
  aliases = aliases || [];
  const data = {
    likes: [],
    reposts: [],
    replies: [],

  if (!webmentions) {
    return data;

  const forThisPage = webmentions
    .filter(isThisUrl(this.page.url, ...aliases))

  data.likes = forThisPage.filter(isLike);
  data.reposts = forThisPage.filter((m) => isRepost(m) || isMention(m));
  data.replies = forThisPage.filter(isReply);

  return data;

I like it when my code makes little sentences: data.likes = forThisPage.filter(isLike); seems nice and clear. I’m also insulated from my own misunderstanding of the webmention.io format: should I need to change anything I can change the isLike function only.

In order to figure out what webmentions I received for this page, I have to filter my webmentions by URL. Eleventy calls its filters within a context that makes this.page available, which means I don’t have to pass in the page.url when I invoke the filter. Kinda nice, but kinda magical. I’m a big fan of magic when it comes from a wizard, less so when it comes from my code – but I’ll keep it for now.

On my site I do this thing where I can add an aliases key to my front matter and it’ll add an entry into my _redirects file for Netlify for that file. I account for that here; I don’t want to lose any interactions on the previous page so I pass those aliases right in.

Here’s the isThisUrl function:

const isThisUrl =
  (...urls) =>
  (mention) =>
    urls.map((u) => `https://drhayes.io${u}`).includes(mention['wm-target']);

This is honestly stretching my limit for a “one-line” arrow function and will probably become a regular ol’ function in the near future.

The other predicates are similar:

function makePredicate(propValue) {
  return function (mention) {
    return mention.type === propValue;

const isValid = (mention) =>
  mention.author &&
  mention.author.name &&
  mention.published &&
  mention.content &&
  // Not sure if this is correct?
const isLike = makePredicate('like-of');
const isMention = makePredicate('mention-of');
const isRepost = makePredicate('repost-of');
const isReply = makePredicate('in-reply-to');

For isValid I’m aping some of the other code I’ve seen and validating that the mention will work on my site. I’m pretty sure a webmention without those fields is still valid, but it’d be hard to meaningfully display it on my site.

…but what if I did this myself?

But hold on. Netlify has these cool function things so I could spend some time figuring out how to receive webmentions using those. I like keeping things in house, and I like writing code (who doesn’t)… but I also like not worrying about my site going down. I’m on the fence about this approach.

I’m trying to break myself of the code habit and trying to create a solution habit. And webmention.io is a perfectly fine solution. My goal is not to make a webmention web service, it’s to be able to display webmentions on my site. Writing my own thing doesn’t serve that goal.

Moving on. 😼

Caching… or maybe it’s Storing

I noticed a couple of implementations caching the results returned from webmention.io.

I’m interested! Especially if webmention.io ever goes down, then poof go all my webmentions. When developing locally I’m entirely dependent on 11ty/eleventy-fetch caching things on my computer while I’m messing around with my site; that’s good enough for the short term. But I’m at a loss, longer term.

Netlify builds my site, so I could cache them up there I guess? I found a couple of utilities to handle something like that for me:

…but that’s not really what I’m thinking of – I don’t want them available for the next build, I want them available for the life of the site. And I think I’m okay with Eleventy’s fetch cache disappearing from build to build on Netlify. At least I am until I notice all my webmentions are gone. 🙀

I did find this post about saving webmentions back to the site’s git repo as they’re received that I could probably copy and paste right over mine for inspiration for my own site. I appreciate how elegant his approach is and how he ends up with what I want: a git repo full of webmentions. I’m not sure I care as much about the moderation side of things, but I’ll probably change my mind after a few too many rotten comments.

A project for future David.


My site is now listening for webmentions and will display any that it gets in a timely fashion thanks to GitHub workflows, a 23 */2 * * * cron expression, and Netlify build hooks.

I’d really like it if my site could send or publish webmentions as well: every time I link to another page it’d be great if the other person knew I was giving them credit for the work they’ve done. I am truly standing on the shoulders of giants.

Speaking of giants in webdev: the inimitable Remy Sharp has done it again and created yet another incredibly useful tool, webmention.app.

Lots of sites recommend using webmention.app and calling it a day. But I like to ice skate uphill, so I dug in a bit and found CodeFoodPixels/netlify-plugin-webmentions! It uses @remy/webmention as a library (the same code that powers webmention.app) but runs “locally” in the build at Netlify. That appeals! By default it searches the most recent post based on the feed for your site.

Well, sort of. I tested the wm command locally (I figure the binary would work a lot like the Netlify plugin) by giving it my feed.xml file[1] and it pooped out on entries in my feed that came early textually – no date sorting evident. That’s not great for my purposes. But I do very much appreciate that it does a dry run by default to let you check your answers. Turns out I can run it directly on the generated HTML of my post (such a great tool) and that works great. I could figure out the most recent piece on my site and run it on that as part of my build, maybe.

But, apparently, brid.gy will also do publishing? All I’d need to do there is send brid.gy a webmention (we must go deeper) and they’d scan my post looking for mention-able things. But how would I know which posts to tell brid.gy about?

While both of these would work they’re not quite satisfying. What if I change something other than the most recent post on my site? I try to keep my updated frontmatter… well, updated. Is that good enough? Maybe it’s not perfect, but that’s okay.

But… I also don’t want to send webmentions for things every time I build my site if I’ve already sent webmentions. I am forever tinkering with my site. I’d hate to keep blasting out webmentions for a blog post because I’m tweaking the colors or the layout. I’m much less sure of how to do that if I don’t save the last updated date somewhere persistent.

This problem feels isomorphic to storing my webmentions in the repo. Surely they’re related. But I don’t want to rush to solve that one just yet.

So I’m going to kick that can down the road and run notify brid.gy manually from my desktop when I publish a new thing. I’ll treat it as motivation to solve this problem. When I do solve this problem I’ll make sure to write about it.

…and I’m done!

And that’s it! Webmentions are printing out on my site and I know how to send them to other sites when I publish.

For the future:

And speaking of webmentions… here’s my social.lol post about this page. Have at it!

  1. Have you seen how pretty it looks? ↩︎

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