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Computed Data New in v0.11.0 Jump to heading

At the end of the Data Cascade you may want to inject Data properties into your data object that are based on other data values. To do that you can use the new eleventyComputed feature.

It is important to note that Computed Data is computed right before templates are rendered. Therefore Computed Data cannot be used to modify the special data properties used to configure templates (e.g. layout, pagination, tags etc.). These restrictions may be relaxed over time.

Real World Example Jump to heading

Say you want to use Eleventy’s Navigation Plugin to create a navigation menu for your site. This plugin relies on the eleventyNavigation object to be set. You don’t necessarily want to set this object manually in front matter in each individual source file. This is where Computed Data comes in!

Consider a blog post with the following front matter format:

Filename posts/my-page-title.md
---
title: My Page Title
parent: My Parent Key
---

If this file is generated by a Content Management System (like Netlify CMS, in my particular case), I may not be able to (or want to) create the eleventyNavigation object for each of them. I would also not be able to just dump a standard Data Directory File in there either (that would be useful for setting defaults but we don’t want the same values for every markdown file). eleventyNavigation properties must be set based on other data properties.

Instead, I created this Data Directory File using eleventyComputed:

Filename posts/posts.11tydata.js
module.exports = {
  eleventyComputed: {
    eleventyNavigation: {
      key: data => data.title,
      parent: data => data.parent
    }
  }
};
If you want to use a JavaScript function for your eleventyComputed properties, you must use either JavaScript front matter or a JavaScript data file (template, directory, or global). YAML and JSON do not support JavaScript functions.

The resulting data for each posts/*.md file when processed by Eleventy has the following structure:

Data Cascade for posts/my-page-title.md
{
  "title": "My Page Title",
  "parent": "My Parent Key",
  "eleventyNavigation": {
    "key": "My Page Title",
    "parent": "My Parent Key"
  }
}

If I wanted this data to be computed for all files, I could instead create the following eleventyComputed.js global data file.

module.exports = {
  eleventyNavigation: {
    key: data => data.title,
    parent: data => data.parent
  }
};

Using JavaScript Jump to heading

Use any arbitrary JavaScript for an eleventyComputed property. Note that JavaScript functions require either JavaScript front matter or a JavaScript data file (template, directory, or global). YAML and JSON do not support JavaScript functions.

Here’s a bunch of examples:

module.exports = {
  eleventyComputed: {
    myTemplateString: "This is assumed to be a template string!",
    myString: data => "This is a string!",
    myFunction: data => `This is a string using ${data.someValue}.`,
    myAsyncFunction: async data => await someAsyncThing(),
    myPromise: data => {
      return new Promise(resolve => {
        setTimeout(() => resolve("100ms DELAYED HELLO"), 100);
      })
    }
  }
};

Using a Template String Jump to heading

If you want to use eleventyComputed in YAML front matter, you can use the template syntax string that matches the syntax of the template.

This is how permalink works, if you’re already familiar with that.

Consider our first example, but using Nunjucks (this example is also valid Liquid syntax).

Filename posts/my-page-title.njk
---
title: My Page Title
parent: My Parent Key
eleventyComputed:
  eleventyNavigation:
    key: "{{ title }}"
    parent: "{{ parent }}"
---

The above would also resolve to the same Data Cascade:

Data Cascade for posts/my-page-title.njk
{
  "title": "My Page Title",
  "parent": "My Parent Key",
  "eleventyNavigation": {
    "key": "My Page Title",
    "parent": "My Parent Key"
  }
}
Template syntax is definitely slower than the “Using JavaScript” methods above.
This would also work in JSON data files or any other data file type in the cascade, just keep in mind that the template syntax must match the template syntax that it eventually winds up with in the Data Cascade.

Advanced Details Jump to heading

We put a lot of work into making this feature as easy to use as possible. Most of these details shouldn’t matter to you as it should Just Work™. But here’s a few things we thought of already and handle in a good way:

---
key: My Key
eleventyComputed:
  key: "This Is {{key}}"
---

Declaring Your Dependencies Jump to heading

We do try our best to automatically detect dependencies between eleventyComputed keys, but it isn’t always 100% accurate—especially if you include conditional logic that only uses another Computed Data key inside of a conditional block. To workaround this issue, you can always declare your dependencies inside of your callback so that it resolves correctly. To do so, just access the variables that your callback uses in the callback function.

module.exports = {
  eleventyComputed: {
    myValue: () => "Hi",
    myOtherValue: () => "Bye",
    usesAllTheThings: data => {
      // We detect this as a declared dependency
      data.myValue;
      // You can use as many as you want.
      data.myOtherValue;
      // You could use any valid JS syntax to access them.
      [data.myValue, data.myOtherValue];

      return `How are you?`;
    }
  }
};

If you suspect Eleventy has the Computed Data order wrong—you can double check what variables Eleventy detects inside of a Computed Data function in the debug output.

Other pages in Data Cascade: