Your "own" structs ... are they indeed passed by value?

I'd like to check something with a Unityscript expert .... thanks!

Generally in Javascript - as I understand it - if something is a struct it is passed by value. (A new copy is made - if you change it you are doing nothing to the original). Whereas, if something is a class, it's passed by reference. (No copy is made of the data. You are actually changing the values in the original.) [See footnote 1]

My question .. as an everyday matter in UnityScript, you make "structs" or pseudo-structs, like this ...

 class EnormousThing extends System.ValueType
 {
 // huge amounts of stuff here
 }

In fact, is it correct that such "pseudo-structs" are ALSO passed by value?

Another way to express this:

If you use "class EnormousThing" it will be passed by reference ("just as with all classes in Javascript"), whereas, if you use the "pseudo-struct" idiom: "class EnormousThing extends System.ValueType" it will be passed by value (|just as with real structs in Javascript")

is this exactly correct?

{As a further bonus question, in fact, is the only difference in adding "extends System.ValueType" in fact the value-versus-reference behaviour so described? Is that the only reason to add "extends System.ValueType" ..? Is there really any other difference in using that qualifier?}

[Footnote 1] Although - this is a detail - cleverly the reference itself is I believe just a copy, so, if you set it to null you have done nothing to the original. Please completely ignore this issue in the question at hand.