Error: Cannot read property 'forEach' of undefined

EDIT: I debugged this issue, see my comment below.

---

I’ve read up on how to use Contentful migrations and wanted to try it out for the very first time. However, when I was running my migration script I got the error Cannot read property 'forEach' of undefined:

🚨  Migration unsuccessful
Unexpected token C in JSON at position 0

🚨  Error: Cannot read property 'forEach' of undefined

  TypeError: Cannot read property 'forEach' of undefined
      at run ([…]/node_modules/contentful-migration/built/bin/cli.js:166:24)
      at processTicksAndRejections (internal/process/task_queues.js:94:5)

The reason why I’m creating an issue here is because I believe that this error is in fact not coming from my migration script. According to the stack trace the error is coming from here: https://github.com/contentful/contentful-migration/blob/be850708122f9cdd5a132d75711fa278bd0ca543/src/bin/cli.ts#L187

This issue seems to be related to https://github.com/contentful/contentful-migration/issues/183. However, #183 was solved by using a different technology (the python content management SDK) to accomplish the goal. I’d rather want this tool here to work though.

Expected Behavior

Either a successful migration or an error message telling what exactly was wrong with my migration script.

Actual Behavior

Inactionable error message / error within the tool itself.

Possible Solution

Either this is a bug in the contentful-migration CLI that needs to be fixed or the error messages should become more helpful / actionable.

Steps to Reproduce

I don’t want to publicly post our space ID, but also don’t want to go through the effort of creating a separate repo for showcasing the error. So, I’ll just post the migration script in case this is of any help:

// migrations/01-lowercase-slug.js

module.exports = function(migration) {
  const transformEntryForLocale = fromFields => {
    const prevSlug = fromFields.slug
    const newSlug =
      typeof prevSlug === 'string' ? prevSlug.toLowerCase() : prevSlug
    return { slug: newSlug }
  }

  migration.transformEntries({
    contentType: 'crmPage',
    from: ['slug'],
    to: ['slug'],
    shouldPublish: 'preserve',
    transformEntryForLocale,
  })

  migration.transformEntries({
    contentType: 'pcmPage',
    from: ['slug'],
    to: ['slug'],
    shouldPublish: 'preserve',
    transformEntryForLocale,
  })
}

Environment

• Node Version: v12.14.1
• Package Manager Version: yarn 1.21.1
• Operating System: Darwin Olivers-MBP.fritz.box 19.2.0 Darwin Kernel Version 19.2.0: Sat Nov 9 03:47:04 PST 2019; root:xnu-6153.61.1~20/RELEASE_X86_64 x86_64
• Package Version: 1.2.13 (contentful -v)

[phoebeschmidt]

🎉 This issue has been resolved in version 1.0.13 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀

[osartun]

@patrickdmccarthy So, my way of debugging this might not be the most elegant way: I went into my node_modules directory (!), searched for the contentful-migration folder and added some console.logs to it. Later, I also added debugger statements or breakpoints in VSCode and attached the Node debugger to the process (See here: https://code.visualstudio.com/docs/nodejs/nodejs-debugging#_attach-to-node-process-action). The code in my node_modules folder is the built code which obviously looks different than the source code in the git repo here, so I looked into the source code here and compared it to the code I read in my node_modules folder to see where I’m at. The source code is easier to read and to understand, of course.

I hope this can help you! 🙏