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I want to write a cloneDeep function that clones any JS object. There are two distinct problems here:

  1. Self references.
  2. Exotic objects.

I want to focus on the latter. Exotic objects in ECMAScript are listed in this section: 10.4 Built-in Exotic Object Internal Methods and Slots. There, it mentions exotic objects like strings, bound functions, Arrays etc. Specifically, it does not mention RegExp.

So, my expectation was that given RegExp is an ordinary object, if I copy all its own properties (enumerable and non-enumerable) and its prototype, it will behave like the source RegExp object. But that is not true: it fails when I try to access the getter regex.source by explicitly saying it is not a RegExp object.

Digging further, I found that some user-inaccessible internal slots exist in ECMAScript for various objects only to be accessible through their constructors.

This leads me to believe either of the two is true:

  1. RegExp is also an exotic object somehow.
  2. It is an ordinary object but there is some other state I did not copy.

Which one of these is true? Because I am not able to wrap my head around an ordinary object with object-specific internal slots.

Bergi
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Anurag Kalia
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  • how do you plan to copy arbitrary non-enumerable properties, exactly? – Dai Jul 08 '22 at 19:02
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    Anyway, _why are you doing this_? I can understand a need to clone POJOs (simple `object` types with only specifically-assigned value-properties with no constructors nor prototypes) but there simply isn't a need to clone _every_ object in JS, nethertheless as you've discovered it is _impossible_ for userland JS to clone many (or even most?) environment-provided objects like `RegExp` - I think you're wasting your time. – Dai Jul 08 '22 at 19:05

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It is an ordinary object with some internal state that you could not copy.

Being an exotic object only means that it has some extraordinary implementation of the internal methods, not that it doesn't have any internal slots. Many builtin functions - such as the .source getter you describe - still require being called on an object with specific internal slots.

Bergi
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