question about pagination with collection

[btrem]

I'm using a collection of posts by year using this snippet in .eleventy.js:

    eleventyConfig.addCollection('postsByYear', function(collectionApi) {

        let postDate, yyyy;
        let postsByYear = {};
        let posts = collectionApi.getFilteredByGlob('src/posts/**/*.*');

        posts.forEach(function(post) {
            postDate = new Date(post.data.date);
            yyyy = postDate.getFullYear().toString();

            // create a new array key with the year if it doesn't already exist
            if (!postsByYear[yyyy]) {
                postsByYear[yyyy] = [];
            }
            // add post to the year array key
            postsByYear[yyyy].push(post);
        });
        return postsByYear;
    });

Then I have this template in the root of my src directory:

---
pagination:
    data: collections.postsByYear
    size: 1
    alias: year
permalink: /{{ year }}/
---
{% for post in collections.postsByYear[year] | reverse %}
<!-- show post title, date, excerpt, etc. -->
{%- endfor %}

It creates an index of posts by year, e.g. /2021/index.html and /2022/index.html. But there are aspects that I don't understand. For instance, what does the alias year refer to? The data is a collection. Is year the index of an array of posts?

I ask because I am having nothing but trouble trying to extend this to months, e.g. /2022/07/index.html.

[pdehaan]

"But there are aspects that I don't understand. For instance, what does the alias year refer to? The data is a collection. Is year the index of an array of posts?"

It looks like your collections.postsByYear collection returns an Object (where the stringified year is the key, and array of posts is the value). See https://www.11ty.dev/docs/pagination/#paging-an-object

"Eleventy does allow paging objects too. Objects are resolved to pagination arrays using either the Object.keys() or Object.values() JavaScript functions."

So in your case, I think the year alias is one of the keys of the collections.postsByYear object.

Not sure the best way to extend this to year+month, since that'd be some non-trivial nested double pagination.
My first thought would be maybe to copy/paste your code and create a second collection named collections.postsByYearMonth and then set the keys to something like 2022_08 (or 2022/08) with an array value of posts in that year+month combination, and then have to tweak the permalink to get the output file structure that you want.