Error generating template - slugify

[oflavio]

Hello,

I'm totally new to SSG and web programming in general. I followed the tutorial in https://getzeropoint.com/ right up to the point before Adding a CMS but cannot run "npm run dev" without errors.

$npx @11ty/eleventy --version 3.1.2
$node --version v24.11.1
$npm --version 11.6.2

flavio@flaviopc:~/Dropbox/Site/zero$ npm run dev

> zero@2026.1 dev
> npm run 🪐


> zero@2026.1 🪐
> cross-env ELEVENTY_ENV=development eleventy --serve


  🪐 zero
  by oflavio
  │
  ├── 📚 Collections (/src/config/collections.js)
  │      ├── ● pages
  │      └── ● posts
  │
  ├── 🔌 Plugins (/src/config/plugins.js)
  │      ├── ● image
  │      └── ● reusableComponents
  │
  ├── ⏩ Shortcodes (/src/config/shortcodes.js)
  │      └── ● year
  │
  ├── 🔍 Filters  (/src/config/filters.js)
  │      └── ● markdown
  │
  ├── 🔀 Transforms (/src/config/transforms.js)
  │      └── ● beautify
  │
  ├── 🚚 Passthroughs (/src/config/passthroughs.js)
  │      ├── ● fonts
  │      └── ● images
  │
  └── 👀 Watch Targets (/src/config/watchtargets.js)
         └── ● assets

[11ty] Problem writing Eleventy templates:
[11ty] 1. Error generating template page(s) for ./src/content/posts/hello-world (Copy).md. (via EleventyMapPagesError)
[11ty] 2. Cannot read properties of undefined (reading 'slugify') (via TypeError)
[11ty] 
[11ty] Original error stack trace: TypeError: Cannot read properties of undefined (reading 'slugify')
[11ty]     at Object.permalink (file:///home/flavio/Dropbox/Site/zero/src/content/posts/posts.11tydata.js:32:48)
[11ty]     at ComputedDataProxy.findVarsUsed (file:///home/flavio/Dropbox/Site/zero/node_modules/@11ty/eleventy/src/Data/ComputedDataProxy.js:124:9)
[11ty]     at ComputedData.resolveVarOrder (file:///home/flavio/Dropbox/Site/zero/node_modules/@11ty/eleventy/src/Data/ComputedData.js:75:29)
[11ty]     at async ComputedData.setupData (file:///home/flavio/Dropbox/Site/zero/node_modules/@11ty/eleventy/src/Data/ComputedData.js:105:3)
[11ty]     at async Template.addComputedData (file:///home/flavio/Dropbox/Site/zero/node_modules/@11ty/eleventy/src/Template.js:629:4)
[11ty]     at async Template.getTemplates (file:///home/flavio/Dropbox/Site/zero/node_modules/@11ty/eleventy/src/Template.js:800:4)
[11ty]     at async #initDependencyMapEntry (file:///home/flavio/Dropbox/Site/zero/node_modules/@11ty/eleventy/src/TemplateMap.js:192:17)
[11ty]     at async TemplateMap.initDependencyMap (file:///home/flavio/Dropbox/Site/zero/node_modules/@11ty/eleventy/src/TemplateMap.js:186:4)
[11ty]     at async TemplateMap.cache (file:///home/flavio/Dropbox/Site/zero/node_modules/@11ty/eleventy/src/TemplateMap.js:274:3)
[11ty]     at async TemplateWriter._createTemplateMap (file:///home/flavio/Dropbox/Site/zero/node_modules/@11ty/eleventy/src/TemplateWriter.js:352:3)
[11ty] Wrote 0 files in 0.33 seconds (v3.1.2)
[11ty] Eleventy Fatal Error (CLI):
[11ty] 1. Error generating template page(s) for ./src/content/posts/hello-world (Copy).md. (via EleventyMapPagesError)
[11ty] 2. Cannot read properties of undefined (reading 'slugify') (via TypeError)
[11ty] 
[11ty] Original error stack trace: TypeError: Cannot read properties of undefined (reading 'slugify')
[11ty]     at Object.permalink (file:///home/flavio/Dropbox/Site/zero/src/content/posts/posts.11tydata.js:32:48)
[11ty]     at ComputedDataProxy.findVarsUsed (file:///home/flavio/Dropbox/Site/zero/node_modules/@11ty/eleventy/src/Data/ComputedDataProxy.js:124:9)
[11ty]     at ComputedData.resolveVarOrder (file:///home/flavio/Dropbox/Site/zero/node_modules/@11ty/eleventy/src/Data/ComputedData.js:75:29)
[11ty]     at async ComputedData.setupData (file:///home/flavio/Dropbox/Site/zero/node_modules/@11ty/eleventy/src/Data/ComputedData.js:105:3)
[11ty]     at async Template.addComputedData (file:///home/flavio/Dropbox/Site/zero/node_modules/@11ty/eleventy/src/Template.js:629:4)
[11ty]     at async Template.getTemplates (file:///home/flavio/Dropbox/Site/zero/node_modules/@11ty/eleventy/src/Template.js:800:4)
[11ty]     at async #initDependencyMapEntry (file:///home/flavio/Dropbox/Site/zero/node_modules/@11ty/eleventy/src/TemplateMap.js:192:17)
[11ty]     at async TemplateMap.initDependencyMap (file:///home/flavio/Dropbox/Site/zero/node_modules/@11ty/eleventy/src/TemplateMap.js:186:4)
[11ty]     at async TemplateMap.cache (file:///home/flavio/Dropbox/Site/zero/node_modules/@11ty/eleventy/src/TemplateMap.js:274:3)
[11ty]     at async TemplateWriter._createTemplateMap (file:///home/flavio/Dropbox/Site/zero/node_modules/@11ty/eleventy/src/TemplateWriter.js:352:3)

[MWDelaney]

Hello, @oflavio and I apologize for the delay in responding --it was the holidays.

It looks like the error you're getting is related to the file hello-world (Copy).md. What is the content of this file? If you delete it, does the build work?

[Newman5]

I got this error too. I was able to fix it with CoPilot. Check out my commits as I go thru the process of trying to get the template to work. https://github.com/Newman5/oreo-blog

My goal is to learn 11ty and ultimately get to a CMS to easily write blog posts Thanks @MWDelaney

update: Here's what I found from my chat logs.
I installed slugify and imported it to 'posts.11tydata.js'

The error occurs because data.eleventy.env.filters.slugify is undefined. In Eleventy, filters are not always available in the data cascade like this. The recommended way is to import a slugify function directly or use Eleventy's built-in slugify filter via the global this context (if available), or by importing a package.

To fix this, you should import a slugify package at the top and use it in your permalink function. Here’s how you can fix it:

Then
There was a problem with Nunjucks and Liquid syntax

The Eleventy build now fails due to a syntax error in src/content/blog.njk, specifically:

expected "|" before filter, file:./src/content/blog.njk, line:1, col:73

This means the template syntax is invalid—likely a Liquid template expecting a filter (|) or a syntax issue with how the permalink or pagination is written.

so changed blog.njk to change the syntax to nunjucks

Then
There was liquid in the frontmatter

The build still fails because Eleventy is trying to parse your frontmatter as Liquid, not Nunjucks. The syntax {{ ... }} and ~ string concatenation only works in Nunjucks, but your project is configured to use Liquid for frontmatter.

To fix this, you must use Liquid syntax in the frontmatter, not Nunjucks. For dynamic permalinks and titles in Liquid, use:

String concatenation with {{ ... }} and filters, not ~.
Use {% if ... %} ... {% endif %} for conditionals.
Here’s a Liquid-compatible version for your frontmatter:
permalink: /blog/{% if pagination.pageNumber > 0 %}/page/{{ pagination.pageNumber | plus: 1 }}{% endif %}/index.html
eleventyComputed:
  title: Blog{% if pagination.pageNumber > 0 %} (Page {{ pagination.pageNumber | plus: 1 }}){% endif %}

... I see where this was a bit Vibe coding funky - why switch back to Liquid? Why didn't I catch that at the time? hmmmm....

Then

Your Eleventy build is still failing because the frontmatter in [blog.njk](vscode-file://vscode-app/c:/Users/nslan/AppData/Local/Programs/Microsoft%20VS%20Code/resources/app/out/vs/code/electron-browser/workbench/workbench.html) is being parsed as Liquid, not Nunjucks—even though your templates use Nunjucks. This is a common Eleventy issue: frontmatter is always parsed as Liquid unless you explicitly configure otherwise.

To fully migrate your site to Nunjucks and avoid Liquid parsing errors, you need to:

Change Eleventy’s template engine for frontmatter from Liquid to Nunjucks in your Eleventy config (usually [.eleventy.js](vscode-file://vscode-app/c:/Users/nslan/AppData/Local/Programs/Microsoft%20VS%20Code/resources/app/out/vs/code/electron-browser/workbench/workbench.html) or config/config.js).
Ensure all frontmatter and templates use Nunjucks syntax.
Would you like me to update your Eleventy config to set Nunjucks as the default template engine for frontmatter and templates? This will resolve the parsing conflict and allow you to use Nunjucks everywhere.

which looks like :

and that fixed it... I wonder if the slugify and the template stuff are related now. hmmm... I hope this helps.

[MWDelaney]

Thank you both for reporting this issue! #49 should fix this.