Bug: (flat config) ESLint crashes when merging a null value in a config

[bradzacher]

Environment

Node version: v18.16.1
npm version: v9.5.0
Local ESLint version: v8.55.0 (Currently used)
Global ESLint version: Not found
Operating System: darwin 23.1.0

What parser are you using?

Default (Espree)

What did you do?

module.exports = [
  {
    languageOptions: {
      parserOptions: {
        foo: {},
      }
    },
  },
  {
    languageOptions: {
      parserOptions: {
        foo: null,
      }
    },
  },
]

What did you expect to happen?

The second parserOptions.foo should override the first - which is the way it used to work with .eslintrc configs.

What actually happened?

Oops! Something went wrong! :(

ESLint: 8.55.0

TypeError: Cannot convert undefined or null to object
    at Function.keys (<anonymous>)
    at deepMerge (<root>/node_modules/eslint/lib/config/flat-config-schema.js:91:30)
    at deepMerge (<root>/node_modules/eslint/lib/config/flat-config-schema.js:105:27)
    at ObjectSchema.merge (<root>/node_modules/eslint/lib/config/flat-config-schema.js:271:16)
    at <root>/node_modules/@humanwhocodes/object-schema/src/object-schema.js:244:54
    at Array.reduce (<anonymous>)
    at ObjectSchema.merge (<root>/node_modules/@humanwhocodes/object-schema/src/object-schema.js:237:24)
    at ObjectSchema.merge (<root>/node_modules/@humanwhocodes/object-schema/src/object-schema.js:176:39)
    at <root>/node_modules/@humanwhocodes/object-schema/src/object-schema.js:244:54
    at Array.reduce (<anonymous>)

Link to Minimal Reproducible Example

https://github.com/bradzacher/eslint-flat-config-bugs/tree/main/null-merge-crash

Participation

• I am willing to submit a pull request for this issue.

Additional comments

In typescript-eslint we have a convenience config which can be used to disable type-aware linting on files i.e. someone can use { files: ["*.js"], extends: ['plugin:@typescript-eslint/disable-type-checked'] } to override their base config and disable type-aware linting.

In eslintrc files this works as expected. In flat configs this crashes.

[fasttime]

Thanks for the report! There happens to be an ongoing discussion about this problem in another issue: #17752 (comment).

[nzakas]

We should definitely be duplicating the merge behavior of eslintrc for this case, so marking as accepted.

[JoshuaKGoldberg]

Should this be blocked on #17752 being merged? That PR also changes the deepMerge function and adds tests. I suspect it'd be easier to wait to implement.

[mdjermanovic]

#17752 has been merged now.

[fasttime]

I would like to work on this, as I'm already familiar with the relevant code.

[fasttime]

I've drafted a PR here to make the way merging works more similar to eslintrc. I've also included a list of cases that would be affected by the changes, which I will keep up-to-date if we decide make more changes. These should make it easier to discuss the details. Please, have a look.

[bradzacher]

Question - I see that you have just done a v9 alpha release. Will this fix be released for v8? The reason being that this is blocking us from supporting flat configs and so is also blocking many of our users from switching to flat configs.

Would be good to get this out before v9 so that we can help users migrate ahead of v9 to ease migration pains.

[fasttime]

AFAIK no new v8 versions are planned. v8.56.0 is expected to be the last one: https://github.com/eslint/tsc-meetings/blob/main/notes/2023/2023-12-14-transcript.md?plain=1#L77-L79.

[bradzacher]

This is a real problem for us as mentioned. There's a non-trivial number of our users that are stuck then and are thus going to be hit with a really painful migration to v9 as they will have to deal with the flat config migration AND whatever other breaking changes are shipping with the major.

I would strongly urge you guys to reconsider and backport just bug fixes to v8 to aid with migrations.

As mentioned - we can't migrate the @typescript-eslint repo to a flat config because of this bug - which is blocking us from releasing our own flat config support.

It would be great if we could release official support from our project to help people migrate early given that we currently support around 70% of eslint users.

[fasttime]

@nzakas @mdjermanovic anything we can do to help with the typescript-eslint migration?

[bradzacher]

Just pinging back on this in the new year. Is this something we can look into for backporting to v8?

We have users playing around with v9-alpha and finding crashes and we can't even begin to consider handling that until we have flat configs working.

But we can't get flat configs working without the fix.

[fasttime]

@bradzacher To request a new v8 release maybe it would be better if you open a new issue. A new latest release would be more work than just backporting that PR given our current setup and workflow. I can't say if that request would be accepted, but clearly the discussion would be more visible.

[woutermont]

@bradzacher, is this issue only a problem for the convenience config, or does it pop up elsewhere too?

To unblock the stalemate, might it be an idea to temporarily use false instead of null to communicate that disabling override?

[nzakas]

@woutermont please see discussion on #17966