📦🔥：keep non-plain objects whole in deep copies

[DerekNonGeneric]

Requested by DerekNonGeneric

Before: deepAssign({}, { fn, when: new Date(0), m: new Map(), re: /x/g }) returned { fn: {}, when: {}, m: {}, re: {} }. deepMixin did the same. deepMerge({ d: new Date(0) }, { d: new Date(5) }) kept the old date, because it merged one Date into the other and a Date has no own properties to copy.

After: functions, Dates, Maps, RegExps and class instances are assigned by reference, and only plain objects are copied or merged property by property.

This is the rule _copyDeep already follows inside arrays.

How: _mixin and deepMerge now check the source value with isPlainObject before they recurse. Tests for all three helpers fail on main and pass here.

Summary by CodeRabbit

• Bug Fixes
  • Deep assignment and mixing now recursively copy plain objects and arrays while preserving other values—such as dates, maps, functions, and class instances—by reference.
  • Deep merging now replaces existing target values with source values such as dates and maps, rather than merging into those values.
• Documentation
  • Clarified how deep assignment, mixing, and merging handle nested values.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Essentials

Run ID: 0ad6fc3a-7554-40ae-9fa5-3d19908e3ac4

📥 Commits

Reviewing files that changed from the base of the PR and between 45679ff and 81d778b.

📒 Files selected for processing (8)

• .changeset/deep-copies-keep-objects.md
• packages/util-object/src/_internal/_mixin.ts
• packages/util-object/src/helpers/deep-assign.ts
• packages/util-object/src/helpers/deep-merge.ts
• packages/util-object/src/helpers/deep-mixin.ts
• packages/util-object/test/helpers/deep-assign.ts
• packages/util-object/test/helpers/deep-merge.ts
• packages/util-object/test/helpers/deep-mixin.ts

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📝 Walkthrough

Walkthrough

Deep assignment and mixin now recursively copy nested plain objects and arrays while retaining other values by reference. Deep merge now recursively merges plain source objects and replaces target values with other source values.

Changes

Deep copy and merge behavior

Layer / File(s)	Summary
Deep assignment and mixin behavior .changeset/deep-copies-keep-objects.md, packages/util-object/src/_internal/_mixin.ts, packages/util-object/src/helpers/deep-assign.ts, packages/util-object/src/helpers/deep-mixin.ts, packages/util-object/test/helpers/deep-assign.ts, packages/util-object/test/helpers/deep-mixin.ts	The shared mixin implementation recurses into plain objects. Documentation and tests specify that functions and non-plain objects, including dates, maps, regular expressions, and class instances, are assigned by reference.
Deep merge behavior packages/util-object/src/helpers/deep-merge.ts, packages/util-object/test/helpers/deep-merge.ts	Deep merge now recurses only for plain source objects. Other source values replace target values. Tests verify that source Date and Map instances replace the target values.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Merge Risk: ⚪ Minimal · up to 81d77

The changes consistently preserve non-plain values by reference while retaining recursive handling for plain objects. No concrete merge-blocking risk is established; merge after normal checks pass.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 7 files. (1 skipped: 1 …
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: non-plain objects remain intact during deep-copy operations. The emojis add noise but do not obscure the meaning.

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