Today I'm following up (heh, get it) on the series I started yesterday on interesting use cases for your Jamstack site's front matter. In yesterday's post, I described how to use front matter to define a list of "related posts" to a blog post. Today's post is a natural follow-up to that one and deals with... well follow-ups!

When I was describing the behavior of related posts in my demo yesterday, I mentioned that the "relationship" would be one way. So post A may be related to B and C, and we would render that, but it would not automatically create a relationship from B to A. As I said in the post, I can definitely see people disagreeing with that, but I had in mind a more natural way for an older post to be related to newer content, a follow-up.

Defining the Follow-Up

So unlike the previous post where I had to define an array in front matter, this time I'm going to use a simple key/value pair. Here's an example:

followup: /posts/beta/

As discussed in the last post, I need some way to link to another post and the URL seems like a natural fit. That was easy!

Displaying the Follow-Up

Now that I've got a way to define a follow-up, let's look at how to display it. As with the previous post, my example code uses a post layout where I define how blog posts are rendered. Previous, the content above where the blog post would go was rather simple:

<h2>{{ title }}</h2>
<p><i>Published {{ date | dtFormat }}</i></p>

In my opinion, if I've written a follow-up to a post it should be noted on top, so I'm going to add it there:

<h2>{{ title }}</h2>
<p><i>Published {{ date | dtFormat }}</i></p>
{% if followup %}
	{% assign followupPost = followup | getByURL: collections.posts %}
	<p><strong>Followup:</strong> <a href="{{ followupPost.url }}">{{ followupPost.data.title }}</a></p>
{% endif %}

The change here is to simply check for the existence of the followup data, and if so, display it. To get the post we'll call a new filter named getByURL. Remember that we need to pass the actual data (the URL) as well as the collection of data to check.

Let's look at that code now.

eleventyConfig.addFilter("getByURL", function(url, posts) {
    return posts.reduce((prev, p) => {
        if(p.data.page.url === url) return p;
        else return prev;
    });
});

I got fancy and made use of reduce to transform the array of posts to one object based on the URL. Nice and simple. Here's an example of it being rendered:

And that's it! Yes, this is a pretty trivial use of front matter, but as I said, I think it pairs well with the previous post. You can find the source code for this tip here: https://github.com/cfjedimaster/eleventy-demos/tree/master/funwithfrontmatter2

Extra Credit

Ok, if you want, you can stop reading now. In my original post, I mentioned how I wanted this series of posts to be Jamstack engine agnostic, but that I'd be using Eleventy as it's my favorite. I want to share a quick tip that's specific to Eleventy and this post.

If you remember from yesterday's post, I had an array of URLs pointing to related content. The filter to transform those URLs into an array of pages looked like so:

eleventyConfig.addFilter("getRelated", function(relatedPosts, posts) {
    let related = [];
    posts.forEach(p => {
        if(relatedPosts.includes(p.data.page.url)) related.push(p);
    });
    return related;
});

In today's post, I'm doing similar logic, but for just one URL. One nice thing that Eleventy supports is the ability for one filter to call another, and that means I can rewrite the above logic (assuming I've got a site using both) to be a bit simpler:

eleventyConfig.addFilter("getRelated", function(relatedPosts, posts) {
    return relatedPosts.map(p => eleventyConfig.getFilter('getByURL')(p, posts));
});

Now I simply map the passed-in array to an array of pages using the getByURL filter. This example can be found in the same repository linked above.