Bug with Filters in Computed Data

[nhoizey]

Describe the bug

I tried to replace all renderData in my templates with eleventyComputed, but the build now doesn't work.

To Reproduce

I used to have this YFM in this template:
https://github.com/nhoizey/nicolas-hoizey.com/blob/master/src/articles/index-months.njk

---
layout: page
pagination:
  data: collections.articlesByMonth
  size: 1
  alias: month
permalink: "articles/{{ month }}/"
renderData:
  title: Articles published in {{ month | monthString }}
---

…

I tried to replace renderData with eleventyComputed, but it gives me this error:

Problem writing Eleventy templates: (more in DEBUG output)
> Having trouble rendering njk template ./src/articles/index-months.njk

`TemplateContentRenderError` was thrown
> (./src/articles/index-months.njk)
  TypeError: Cannot read property 'replace' of undefined

`Template render error` was thrown:
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! [email protected] eleventy:build: `cross-env NODE_ENV=production eleventy`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the [email protected] eleventy:build script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/nhoizey/.npm/_logs/2020-03-30T09_26_27_412Z-debug.log

I guess it comes from the alias because the error Cannot read property 'replace' of undefined comes from the monthString filter:
https://github.com/nhoizey/nicolas-hoizey.com/blob/master/src/_11ty/filters/dates.js#L7-L11

monthString: (month) => {
  // transforms "2020/02" into "February 2020"
  let fullDate = `${month.replace('/', '-')}-01T10:00:00.000Z`;
  return moment(fullDate).format('MMMM YYYY');
},

Expected behavior

I thought eleventyComputed was supposed to replace renderData completely.

Environment:

• OS and Version: macOS 10.15.3
• Eleventy Version 0.11.0-beta-3

[zachleat]

What does the new failing one look like?

[nhoizey]

@zachleat do you mean the YFM that doesn't work?

Same as the other with just renderData replaced by eleventyComputed:

---
layout: page
pagination:
  data: collections.articlesByMonth
  size: 1
  alias: month
permalink: "articles/{{ month }}/"
eleventyComputed:
  title: Articles published in {{ month | monthString }}
---

…

[jasonday]

I read somewhere that the title should be in "" when using eleventyComputed, but I can't get eleventyComputed working either.