feat(collections): option to merge undefined in deepMerge

[WWRS]

resolves #4752

Default is "replace", which is the existing behavior.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.22%. Comparing base (b5a5fe4) to head (88605c0).
Report is 53 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6522   +/-   ##
=======================================
  Coverage   95.21%   95.22%           
=======================================
  Files         577      577           
  Lines       43381    43385    +4     
  Branches     6487     6489    +2     
=======================================
+ Hits        41307    41312    +5     
+ Misses       2034     2033    -1     
  Partials       40       40           

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[ry]

strange that the key is a keyword undefined

[WWRS]

That's true. Maybe undefineds would be better?

[kt3k]

@WWRS Sorry for late reply, but undefineds sounds a bit strange to me. Because this affects only when undefined appears as object property, how about undefinedProp, undefinedProperty, or propertyUndefined? cc @ry

[WWRS]

Just above, the options are named arrays, sets, and maps, so I think undefineds does match with those most closely

[kt3k]

I see the point, but the usage of the word undefineds seems very rare compared to sets, maps, arrays. I'm afraid this might be confusing to the users. (I personally prefer the original undefined over undefineds. undefined is not JavaScript keyword (ref), but just predefined variable name. I personally disagree with @ry's concern..)