webmention-handler

webmention-handler is a Node.js handler for the 2017 W3C Recomendation Spec of Webmention. It is written in TypeScript and includes full type definitions.

What is a webmention

A Webmention is a notification that one URL links to another. For example, Alice writes an interesting post on her blog. Bob then writes a response to her post on his own site, linking back to Alice's original post. Bob's publishing software sends a Webmention to Alice notifying that her article was replied to, and Alice's software can show that reply as a comment, like, repost or other relevant type on the original post.

Installation

Install with NPM:

npm install webmention-handler --save

Setup

To begin with, you should create a webmention storage instance. While there is a default implementation, it is not fit for production environments and is included for demonstration purposes only. To create instance of the example storage class, the following code can be used.

import { LocalWebMentionStorage } from 'webmention-handler';

const storage = new LocalWebMentionStorage();

Different storage implementations may take different parameters so please read the documentation for the chosen implementation. With that, however, you can create an instance of your the webmention handler.

import { WebMentionHandler } from 'webmention-handler';

const options = {
  supportedHosts: ['localhost'] // The domain of any websites this handler should support
  storageHandler: storage, // Your storage handler instance
  requiredProtocol: 'https' // Not required, but highly recommended to only allow https mentions
};

export const webMentionHandler = new WebMentionHandler(options)

Usage

Example Express endpoint to accept webmentions:

// Regular Express setup
import express from 'express';
import bodyParser from 'body-parser';
import webMentionHandler from 'path/to/your/webMentionHandler/file.ts';

const app = express();

// Webmentions require the ability to parse Form encoded data from a post request.
app.use(bodyParser.urlencoded({ extended: true }));

app.post('/webmentions', async function(req, res) {
  try {
    // Pass the source and target of the request to the handler.
    // No need to run validation on it as this will be validated by the handler.
    const recommendedResponse = await webMentionHandler.addPendingMention(req.body.source, req.body.target);

    // The response code is dictated by the specification but not enforced by the handler
    // in case you need to action your own out-of-spec actions. The response body should be human-readable
    res.status(recommendedResponse.code).send('accepted');
  } catch (error) {
    res.status(400).send(error.message);
  }
});

// Open your Express server
app.listen(8080);

Once your endpoint is set up, you can set up a regular job with the following code to convert pending mentionNotifications into mention objects in your database that you can use.

import webMentionHandler from 'path/to/your/webMentionHandler/file.ts';

setInterval(() => webMentionHandler.processPendingMentions(), 300000);

I'd highly recommend processing mentions in an ephemeral cloud function, rather than a setInterval on your main Node application, so that you don't accidentally leak the IP of your server to third parties. Especially if you aren't using the whitelist option for source hosts.

You can now grab the type of mention that you need for any given page.

import webMentionHandler from 'path/to/your/webMentionHandler/file.ts';

const likes = webMentionHandler.getMentionsForPage('/blog/example-post', 'like');
const replies = webMentionHandler.getMentionsForPage('/blog/example-post', 'reply');

Writing custom storage handlers

An example storage handler can be found in the local storage class. It implements the IWebMentionStorage as any other storage handler should also. This ensures compatibility between storage handlers and allows users to switch between different storage handlers at will without updating the rest of their codebase.

Functions

The following functions must be implemented in your storage handler class if you are not using TypesScript.

Function	Arguments	Returns	Explanation
addPendingMention	mention: SimpleMention	Promise<SimpleMention>	Allows the web mention handler to add a new pending mention that needs to be handled
getNextPendingMentions	N/A	Promise<SimpleMention[]>	Fetches a number of pending mentions to be bulk processed. Any configured limits on the number of pending mentions to fetch should be set in the constructor via an options object rather than passed in to this function directly.
getMentionsForPage	page: string, type?: string	Promise<Mention[]>	Gets mentions based on a given target. Has an optional type parameter that allows mentions to be filtered on type. eg. Only Comments or Likes`
storeMentionForPage	page: string, mention: Mention	Promise<Mention>	This function will store a mention on the given target. If you need to access the type of the mention, you can find that on the mention.type property.
deleteMention	mention: SimpleMention	Promise<null>	This function should delete any processed mentions for a given target from a given source. It should always return null and should not error in the event that no mentions are found.

Available storage libraries

Database/storage engine	GitHub Repo	NPM package
MongoDB	vandie/webmention-handler-mongo	webmention-handler-mongodb