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Sia Karamalegos
Sia Karamalegos

Posted on • Updated on • Originally published at

An In-Depth Tutorial of Webmentions + Eleventy

I am a huge fan of the static site generator Eleventy so far, and I was super excited to try out Webmentions with them.

Webmention is a web standard for mentions and conversations across the web, a powerful building block that is used for a growing federated network of comments, likes, reposts, and other rich interactions across the decentralized social web.

They are a cool tool for enabling social interactions when you host your own content. Max Böck wrote an excellent post, Static Indieweb pt2: Using Webmentions, which walks through his implementation. He also created an Eleventy starter, eleventy-webmentions, which is a basic starter template with webmentions support.

So why am I writing this post? Sadly, I started with the eleventy-base-blog, and didn't notice the eleventy-webmentions starter until after I had already built my site. I also struggled to fully build out the functionality, partly because I'm still an Eleventy n00b. So I wanted to share the detailed steps I used in the hopes that it will help more of you join the Indie Web.

The perspective of this post is adding webmentions to an Eleventy site after the fact. The files, folders, and config architecture match the eleventy-base-blog, but you can likely use this as a starting point for any Eleventy site. Make sure you watch out for spots where your analogous architecture may be different.

The code in this post is a mash up of Max Böck's original post and personal site, the eleventy-webmentions starter, Zach Leatherman's personal site, and the edits I made during my implementation. I am hugely grateful for their work, as I never would have gotten this far without it.

Step 1: Sign up for webmentions

First, we need to sign up with, the third-party service that lets us use the power of webmentions on static sites.

  1. Set up IndieAuth so that webmention will know that you control your domain. Follow the setup instructions on their site.
  2. Go to
  3. Enter your website's URL in the "Web Sign-In" input, and click "Sign in".

If your sign in was successful, you should be directed to the webmentions dashboard where you will be given two <link> tags. You should put these in the <head> of your website:

<!-- _includes/layouts/base.njk -->
<link rel="webmention" href="" />
<link rel="pingback" href="" />
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You'll also be given an API key. We want to safely store that in our local environment variables. Add dotenv for easily getting and setting env variables:

$ npm install dotenv
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Create a .env file in the root of your project, and add your API key

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Don't forget to add it to your .gitignore file. While we are here, let's add the _cache/ folder which will be created when we first fetch webmentions:

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You probably want some content in your webmentions. If you use Twitter, Bridgy is a great way to bring in mentions from Twitter. First make sure your website is listed in your profile, then link it.

How it's all going to work

When we run a build with NODE_ENV=production, we are going to fetch new webmentions from the last time we fetched. These will be saved in _cache/webmentions.json. These mentions come from the API.

When we do any build, for each page:

  • From the webmentions cache in _cache/webmentions.json, only keep webmentions that match the URL of the page (for me, this is each blog post).
  • Use a webmentionsByType function to filter for one type (e.g., likes or replies)
  • Use a size function to calculate the count of those mentions by type
  • Render the count with mention type as a heading (e.g., "7 Replies")
  • Render a list of the mentions of that type (e.g., linked Twitter profile pictures representing each like)

Fetching webmentions

First, we need to set up our domain as another property in our _data/metadata.json. Let's also add our root URL for use later:

// _data/metadata.json
  //...other metadata
  "domain": "",
  "url": ""
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Next, we'll add a few more dependencies:

$ npm install lodash node-fetch
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And update our build script to set the NODE_ENV in our package.json:

// package.json
  // ... more config
  "scripts": {
    "build": "NODE_ENV=production npx eleventy",
    // more scripts...
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Now we can focus on the fetch code. Okay, okay, I know this next file is beaucoup long, but I thought it was more difficult to understand out of context. Here are the general steps happening in the code:

  1. Read any mentions from the file cache at _cache/webmentions.json.
  2. If our environment is "production", fetch new webmentions since the last time we fetched. Merge them with the cached ones and save to the cache file. Return the merged set of mentions.
  3. If our envinroment is not "production", return the cached mentions from the file
// _data/webmentions.js
const fs = require('fs')
const fetch = require('node-fetch')
const unionBy = require('lodash/unionBy')
const domain = require('./metadata.json').domain

// Load .env variables with dotenv

// Define Cache Location and API Endpoint
const CACHE_FILE_PATH = '_cache/webmentions.json'
const API = ''
const TOKEN = process.env.WEBMENTION_IO_TOKEN

async function fetchWebmentions(since, perPage = 10000) {
  // If we dont have a domain name or token, abort
  if (!domain || !TOKEN) {
    console.warn('>>> unable to fetch webmentions: missing domain or token')
    return false

  let url = `${API}/mentions.jf2?domain=${domain}&token=${TOKEN}&per-page=${perPage}`
  if (since) url += `&since=${since}` // only fetch new mentions

  const response = await fetch(url)
  if (response.ok) {
    const feed = await response.json()
    console.log(`>>> ${feed.children.length} new webmentions fetched from ${API}`)
    return feed

  return null

// Merge fresh webmentions with cached entries, unique per id
function mergeWebmentions(a, b) {
  return unionBy(a.children, b.children, 'wm-id')

// save combined webmentions in cache file
function writeToCache(data) {
  const dir = '_cache'
  const fileContent = JSON.stringify(data, null, 2)
  // create cache folder if it doesnt exist already
  if (!fs.existsSync(dir)) {
  // write data to cache json file
  fs.writeFile(CACHE_FILE_PATH, fileContent, err => {
    if (err) throw err
    console.log(`>>> webmentions cached to ${CACHE_FILE_PATH}`)

// get cache contents from json file
function readFromCache() {
  if (fs.existsSync(CACHE_FILE_PATH)) {
    const cacheFile = fs.readFileSync(CACHE_FILE_PATH)
    return JSON.parse(cacheFile)

  // no cache found.
  return {
    lastFetched: null,
    children: []

module.exports = async function () {
  console.log('>>> Reading webmentions from cache...');

  const cache = readFromCache()

  if (cache.children.length) {
    console.log(`>>> ${cache.children.length} webmentions loaded from cache`)

  // Only fetch new mentions in production
  if (process.env.NODE_ENV === 'production') {
    console.log('>>> Checking for new webmentions...');
    const feed = await fetchWebmentions(cache.lastFetched)
    if (feed) {
      const webmentions = {
        lastFetched: new Date().toISOString(),
        children: mergeWebmentions(cache, feed)

      return webmentions

  return cache
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Filters for build

Now that we've populated our webmentions cache, we need to use it. First we have to generate the functions, or "filters" that Eleventy will use to build our templates.

First, I like keeping some filters separated from the main Eleventy config so that it doesn't get too bogged down. The separate filters file will define each of our filters in an object. The keys are the filter names and the values are the filter functions. In _11ty/filters.js, add each of our new filter functions:

// _11ty/filters.js
const { DateTime } = require("luxon"); // Already in eleventy-base-blog

module.exports = {
  getWebmentionsForUrl: (webmentions, url) => {
    return webmentions.children.filter(entry => entry['wm-target'] === url)
  size: (mentions) => {
    return !mentions ? 0 : mentions.length
  webmentionsByType: (mentions, mentionType) => {
    return mentions.filter(entry => !!entry[mentionType])
  readableDateFromISO: (dateStr, formatStr = "dd LLL yyyy 'at' hh:mma") => {
    return DateTime.fromISO(dateStr).toFormat(formatStr);
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Now to use these new filters, in our .eleventy.js, we need to loop through the keys of that filters object to add each filter to our Eleventy config:

// .eleventy.js
// ...Other imports
const filters = require('./_11ty/filters')

module.exports = function(eleventyConfig) {
  // Filters
  Object.keys(filters).forEach(filterName => {
    eleventyConfig.addFilter(filterName, filters[filterName])

  // Other configs...
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I do not have a sanitize HTML filter because I noticed the content data has a text field that is already sanitized. Maybe this is new or not true for all webmentions. I'll update this post if I add it in.

Rendering mentions

Now we're ready to put it all together and render our webmentions. I put them at the bottom of each blog post, so in my _includes/layouts/post.njk, I add a new section for the webmentions. Here, we are setting a variable called webmentionUrl to the page's full URL, and passing it into the partial for the webmentions.njk template:

<!-- _includes/layouts/post.njk -->
  {% set webmentionUrl %}{{ page.url | url | absoluteUrl(site.url) }}{% endset %}
  {% include 'webmentions.njk' %}
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Now we can write the webmentions template. In this example, I will show links, retweets, and replies. First, I set all of the variables I will need for rendering in a bit:

<!-- _includes/webmentions.njk -->
  <!-- Filter the cached mentions to only include ones matching the post's url -->
  {% set mentions = webmentions | getWebmentionsForUrl(metadata.url + webmentionUrl) %}
  <!-- Set reposts as mentions that are `repost-of`  -->
  {% set reposts = mentions | webmentionsByType('repost-of') %}
  <!-- Count the total reposts -->
  {% set repostsSize = reposts | size %}
  <!-- Set likes as mentions that are `like-of`  -->
  {% set likes = mentions | webmentionsByType('like-of') %}
  <!-- Count the total likes -->
  {% set likesSize = likes | size %}
  <!-- Set replies as mentions that are `in-reply-to`  -->
  {% set replies = mentions | webmentionsByType('in-reply-to')  %}
  <!-- Count the total replies -->
  {% set repliesSize = replies | size  %}
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With our variables set, we can now use that data for rendering. Here I'll walk through only "replies", but feel free to see a snapshot of how I handled the remaining sets in this gist.

Since replies are more complex than just rendering a photo and link, I call another template to render the individual webmention. Here we render the count of replies and conditionally plural-ify the word "Reply". Then we loop through the reply webmentions to render them with a new nunjucks partial:

<!-- _includes/webmentions.njk -->
<!-- ...setting variables and other markup -->
{% if repliesSize > 0 %}
<div class="webmention-replies">
  <h3>{{ repliesSize }} {% if repliesSize == "1" %}Reply{% else %}Replies{% endif %}</h3>

  {% for webmention in replies %}
    {% include 'webmention.njk' %}
  {% endfor %}
{% endif %}
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Finally, we can render our replies using that new partial for a single reply webmention. Here, if the author has a photo, we show it, otherwise we show an avatar. We also conditionally show their name if it exists, otherwise we show "Anonymous". We use our readableDateFromISO filter to show a human-friendly published date, and finally render the text of the webmention:

<!-- _includes/webmention.njk -->
<article class="webmention" id="webmention-{{ webmention['wm-id'] }}">
  <div class="webmention__meta">
    {% if %}
      {% if %}
      <img src="{{ }}" alt="{{ }}" width="48" height="48" loading="lazy">
      {% else %}
      <img src="{{ '/img/avatar.svg' | url }}" alt="" width="48" height="48">
      {% endif %}
        <a class="h-card u-url" {% if webmention.url %}href="{{ webmention.url }}" {% endif %} target="_blank" rel="noopener noreferrer"><strong class="p-name">{{ }}</strong></a>
    {% else %}
    {% endif %}

    {% if webmention.published %}
        <time class="postlist-date" datetime="{{ webmention.published }}">
            {{ webmention.published | readableDateFromISO }}
    {% endif %}
      {{ webmention.content.text }}
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Bravely jumping into the black hole...

Does it work?!?! We can finally test it out. First run npm run build to generate an initial list of webmentions that is saved to the _cache/webmentions.json file. Then run your local development server and see if it worked! Of course, you'll need to have at least one webmention associated with a post to see anything. 😁

You can see the result of my own implementation at the bottom of the original blog post. Good luck! Let me know how it turns out or if you find in bugs or errors in this post!

Continue your journey by using Microformats

Keith Grant has a great write-up in his article Adding Webmention Support to a Static Site. Check out the "Enhancing with Microformats" section for an explanation and examples.

Additional resources

  • You can find the full code for my site on Github. It will evolve in the future, I'm sure, so you can focus on this commit which has the bulk of my changes for adding webmentions.
  • How I added dotenv support to Netlify is covered in this Stack Overflow answer.
  • How I set up a "cron" job through Github actions to periodically rebuild my site on Netlify (to grab and post new webmentions) is covered in Scheduling Netlify deploys with GitHub Actions.

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