how do I calculate data on collection of game posts

[btrem]

I have a collection of game result posts:

eleventyConfig.addCollection('gamePosts', function(collectionApi) {
    return collectionApi.getFilteredByGlob('src/posts/games/**/*.md');
});

I loop through the collection and show them on a page. I also show game statistics (e.g., win percentage, winning streak, etc.) on that same page. What I have now is a new collection:

eleventyConfig.addCollection('gameStats', function(collectionApi) {
    let gamePosts = collectionApi.getFilteredByGlob('src/posts/games/**/*.md');

    // loop through collection, calculate stats
    // and store in variables, e.g., `streak`, `winPercentage`

    return {
        streak: streak,
        winPercentage: winPercentage
        // there's additional stats, removed here to keep example simple
    };

});

So I'm recreating the collection, which feels wrong. It seems like filter is the more appropriate tool, where I could pass my game post collection to a filter and have it return stats. But then I'd need a filter for each stat, like eleventyConfig.addFilter('gameStreak', function(myGameCollection) /* calculate winning streak */ }) and eleventyConfig.addFilter('winPercent', function(myGameCollection) /* calculate winning percent */ }). But if I go this route, each filter would make 11ty loop over the collection, instead of doing it once and getting all the results.

So what's the best way to calculate data on a collection?

[pdehaan]

Not sure I'm 100% understanding your setup, but you could create a single filter that takes any arbitrary collection as an argument and returns a "stats" object (which sounds easier than creating n filters; one per stat).

  eleventyConfig.addFilter("gameStats", function (collection = []) {
    return collection.reduce((acc, game) => {
      const { points } = game.data;
      acc.winPercentage = points[0] + points[1];
      acc.streak = acc.streak + 1;
      return acc;
    }, { streak: 0 });
  });

Where I created some fake data with front matter like this:

---
title: Golden State Warriors at Utah Jazz
points:
  - 88
  - 85
winner: Golden State Warriors
---

<h1>{{ title }}</h1>

So verrrry terrible stats calculations, but yeah... I also created a src/posts/games/games.11tydata.json which sets tags for all child templates in that directory:

{
  "tags": ["games"]
}

Now I can just refer to collections.games instead of needing something like collectionApi.getFilteredByGlob('src/posts/games/**/*.md').

And my very bad, no good src/index.liquid looks something like this:

---
title: Stats
---
=
{%- assign stats = collections.games | gameStats -%}

<h1>{{ title }}</h1>
<pre>{{ stats | json }}</pre>

<p>Win Streak: {{ stats.streak }}</p>
<p>Win Pct%: {{ stat.winPercentage }}</p>

And the generated HTML looks like:

<h1>Stats</h1>
<pre>{"streak":2,"winPercentage":173}</pre>

<p>Win Streak: 2</p>
<p>Win Pct%: 173</p>

I think one benefit of this is if you have multiple different seasons of games, you could just add new tags in the right subfolders and then loop over "collections.games_23_24 | gameStats" to loop over a subset of games.

[btrem]

Using assign was the part I was missing (well, set, because I use nunjucks). I tend to avoid set because it seems like a recipe for putting logic in templates, but in this case it is the least worst of the three options.

And good idea about subfolders. I'm not ready to tackle that yet, but it might be fun to calculate stat changes over time.

Thanks as always for your help.