gatsby-transformer-remark: Throws an error when constructor is used as a field value

[stephencweiss]

Description

I use a tag system for my website's blog. I recently wrote a post about instantiating classes using a constructor function -- I added this as a tag to the post and gastsby-node broke.

So too did my graphql query on my tags page.

I think this is likely because of some conversion where the runtime is viewing constructor as a reserved word... though this is the only reserved word I've come across with this behavior.

Steps to reproduce

Minimal Reproduction:

• Repo: https://github.com/stephencweiss/gatsby-bug-constructor-tag
• Install deps and start repo gatsby develop -- you should see an error
• Remove line 3 in ./content/boo.md and re-run -- you should see the error disappear

Expected result

I should be able to have a tag of any valid string and not run into the situation where it breaks the build of my site.

Actual result

$ yarn build

 ERROR #11321  PLUGIN

"gatsby-node.js" threw an error while running the createPages lifecycle:

(acc[key] || []).concat is not a function

failed createPages - 0.064s

 ERROR #85901  GRAPHQL

There was an error in your GraphQL query:

(acc[key] || []).concat is not a function

   1 | query usersstephencodeblogsrcpagestagsJsx3450235017 {
   2 |   site {
   3 |     siteMetadata {
   4 |       title
   5 |     }
   6 |   }
   7 |   allMarkdownRemark(limit: 2000) {
>  8 |     group(field: frontmatter___tags) {
     |     ^
   9 |       fieldValue
  10 |       totalCount
  11 |     }
  12 |   }
  13 | }
  14 |

File path: /Users/stephen/code/blog/src/pages/tags.jsx
Url path: /tags/
Plugin: none

Environment

  System:
    OS: macOS 10.15.3
    CPU: (16) x64 Intel(R) Core(TM) i9-9980HK CPU @ 2.40GHz
    Shell: 5.7.1 - /bin/zsh
  Binaries:
    Node: 12.16.1 - ~/.nvm/versions/node/v12.16.1/bin/node
    Yarn: 1.22.4 - /usr/local/bin/yarn
    npm: 6.13.4 - ~/.nvm/versions/node/v12.16.1/bin/npm
  Languages:
    Python: 2.7.16 - /usr/bin/python
  Browsers:
    Chrome: 80.0.3987.149
    Safari: 13.0.5
  npmPackages:
    gatsby: ^2.19.45 => 2.19.45 
    gatsby-source-filesystem: ^2.2.2 => 2.2.2 
    gatsby-transformer-remark: ^2.7.1 => 2.7.1 

[vladar]

The error you see is a runtime GraphQL error with group by frontmatter___tags. If you change grouping to something else - it doesn't break anymore. Still looking into why this happens with frontmatter___tags

[vladar]

Oh, I see what you mean now. Here is what happens:

const a = {}
a[`constructor`] // expected: undefined, got function

Here is a PR with a fix: #22603