fix(mergedeep): preserve array shape when comparing empty targets

[tdabasinskas]

MergeDeep.compareDeep reports the same kind of change in two different JSON shapes depending on whether the target is empty. Comparing rulesets (any top-level array config works the same):

// repo already has a ruleset: additions is an array
compareDeep([{ name: 'other', ... }], [{ name: 'other', ... }, { name: 'new', ... }])
// => { additions: [ { name: 'new', ... } ], ... }

// repo receives its FIRST ruleset: additions is an index-keyed object
compareDeep([], [{ name: 'new', ... }])
// => { additions: { "0": { name: 'new', ... } }, ... }

Two details combine to cause this. On first invocation, top-level arrays are wrapped into { __array: [...] } before the accumulator-type check runs, so Array.isArray(source) is always false and additions initializes as {} (the array branch of that check is effectively dead code). The non-empty-target path recovers because the __array key is processed as a nested array and unwound before returning. But the empty-target early return does Object.assign(additions, s) — spreading an array into a plain object produces the { "0": ..., "1": ... } shape — and returns without the unwind. That early return also omits deletions from its result, unlike every other return path.

This hits anything consuming plan/diff JSON — dry-run reports, PR comments, or tooling parsing the NopCommand actions. A repo receiving its first ruleset reports index-keyed attribute soup instead of one added item, while the second ruleset onward reports a clean array. (Possibly related to the inconsistent-diff reports in #693).

[Copilot]

Pull request overview

This PR fixes MergeDeep.compareDeep so diffs for top-level array configs (e.g. rulesets) are consistent when comparing against an empty target, and ensures the compare result always includes deletions—improving the stability of plan/diff JSON consumed by dry-run reports and PR comments.

Changes:

• Adjusts the empty-target early return in compareDeep to preserve top-level array diff shape for additions, and to always return deletions.
• Adds unit tests covering top-level array shape consistency and presence of deletions in the result.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
lib/mergeDeep.js	Fixes empty-target diff behavior to preserve top-level array shape and include deletions.
test/unit/lib/mergeDeep.test.js	Adds regression tests for consistent top-level array diff shape and inclusion of deletions.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.