As I was working on making my Today I Learned posts first-party members of this very site (they used to live in a separate site on Glitch), I moved over all the posts and placed them in a new folder called til/. But, each post was getting rendered as plain text, with no header or footer! The blog posts that have always been on this site have a layout: key in the frontmatter, but the TIL posts from the other site did not, and there was a lot of them. Too many to edit each one!

As an added complication, I wanted to use a different layout for the posts in til/ than the posts in blog/, the folder housing the non-til-blog posts.

In 11ty there are a lot of ways of defining data for your templates, including configuration data like layouts. The thing I needed for this was a Directory Data File.

Inside til/, I added a file called til.json. (Relative path: posts/til/til.json, matching posts/subdir/subdir.json as shown in the docs). It's contents are pretty bare bones right now:

{
    "layout": "layouts/til-post.njk"
}

After adding this file with these contents, every page generated from the files in the til folder used the til-post layout. (My layouts are in _includes, by Eleventy default, so that layout would be at _includes/layouts/til-post.njk.)