fix(utils): accept null as default option in applyDefault

[Solo-steven]

PR Checklist

• Addresses an existing open issue: fixes Bug: null as default option is failing in applyDefault #6439
• That issue was marked as accepting prs
• Steps in Contributing were taken

Overview

fix issue #6439

[typescript-eslint]

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[codecov]

Codecov Report

Merging #6724 (081e79f) into main (d67fd6d) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6724   +/-   ##
=======================================
  Coverage   87.47%   87.47%           
=======================================
  Files         379      379           
  Lines       13228    13228           
  Branches     3905     3905           
=======================================
  Hits        11571    11571           
  Misses       1279     1279           
  Partials      378      378           

Flag	Coverage Δ
unittest	87.47% <100.00%> (ø)

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Impacted Files	Coverage Δ
packages/utils/src/eslint-utils/deepMerge.ts	89.47% <100.00%> (ø)

[bradzacher]

I guess the question here is whether or not an explicit null should be ignored like this and overridden by the default, or if it should be seen as a value that shouldn't be overridden.

I think an explicit null should not be overridden, personally.

[Solo-steven]

@bradzacher Thanks for your reply, I think another solution is adding one more conditional to isObjectNotArray like

function isObjectNotArray<T extends ObjectLike>(
  obj: unknown | unknown[],
): obj is T {
  return typeof obj === 'object' && obj != null && !Array.isArray(obj);
}

this solution wouldn't overwrite the null value .

But I want to know why we shouldn't overwrite the value, is it cause more errors?

[bradzacher]

But I want to know why we shouldn't overwrite the value, is it cause more errors?

If a user does a config like { foo: null }, and the default is { foo: { bar: '' } }, then we should not override the explicit null with { bar: '' }.
The same can be said for a top-level null. If the user configures a rule like ['error', null], then we should not override that explicit null with whatever the default is.

Instead I think that we should only override undefined, as we currently do for non-top-level values.

[JoshuaKGoldberg]

Nice, thanks for this!

[JoshuaKGoldberg]

Oh 🤦 my GitHub did a weird and I didn't see @bradzacher's comments. Ignore me, and sorry for the force pushing!

[JoshuaKGoldberg]

👋 @bradzacher I think this got dropped from the queue because I did a silly. Do you want to review this one?

It looks good to me, but just making sure.

[JoshuaKGoldberg]

💯 Thanks! And, sorry for the long wait!